drm/i915: Keep track of pwm-related backlight hooks separately

Currently, every different type of backlight hook that i915 supports is
pretty straight forward - you have a backlight, probably through PWM
(but maybe DPCD), with a single set of platform-specific hooks that are
used for controlling it.

HDR backlights, in particular VESA and Intel's HDR backlight
implementations, can end up being more complicated. With Intel's
proprietary interface, HDR backlight controls always run through the
DPCD. When the backlight is in SDR backlight mode however, the driver
may need to bypass the TCON and control the backlight directly through
PWM.

So, in order to support this we'll need to split our backlight callbacks
into two groups: a set of high-level backlight control callbacks in
intel_panel, and an additional set of pwm-specific backlight control
callbacks. This also implies a functional changes for how these
callbacks are used:

* We now keep track of two separate backlight level ranges, one for the
  high-level backlight, and one for the pwm backlight range
* We also keep track of backlight enablement and PWM backlight
  enablement separately
* Since the currently set backlight level might not be the same as the
  currently programmed PWM backlight level, we stop setting
  panel->backlight.level with the currently programmed PWM backlight
  level in panel->backlight.pwm_funcs->setup(). Instead, we rely
  on the higher level backlight control functions to retrieve the
  current PWM backlight level (in this case, intel_pwm_get_backlight()).
  Note that there are still a few PWM backlight setup callbacks that
  do actually need to retrieve the current PWM backlight level, although
  we no longer save this value in panel->backlight.level like before.

Additionally, we drop the call to lpt_get_backlight() in
lpt_setup_backlight(), and avoid unconditionally writing the PWM value that
we get from it and only write it back if we're in CPU mode, and switching
to PCH mode. The reason for this is because in the original codepath for
this, it was expected that the intel_panel_bl_funcs->setup() hook would be
responsible for fetching the initial backlight level. On lpt systems, the
only time we could ever be in PCH backlight mode is during the initial
driver load - meaning that outside of the setup() hook, lpt_get_backlight()
will always be the callback used for retrieving the current backlight
level. After this patch we still need to fetch and write-back the PCH
backlight value if we're switching from CPU mode to PCH, but because
intel_pwm_setup_backlight() will retrieve the backlight level after setup()
using the get() hook, which always ends up being lpt_get_backlight(). Thus
- an additional call to lpt_get_backlight() in lpt_setup_backlight() is
made redundant.

v9:
* Drop the intel_panel_invert_pwm_level() call in lpt_setup_backlight()
* Remove leftover detritus from lpt_setup_backlight()
v8:
* Go back to getting initial brightness level with
  intel_pwm_get_backlight(), the other fix we had was definitely wrong.
v7:
* Use panel->backlight.pwm_funcs->get() to get the backlight level in
  intel_pwm_setup_backlight(), lest we upset lockdep
* Rebase
* Rename intel_panel_sanitize_pwm_level() to intel_panel_invert_pwm_level()
v6:
* Make sure to grab connection_mutex before calling
  intel_pwm_get_backlight() in intel_pwm_setup_backlight()
v5:
* Fix indenting warnings from checkpatch
v4:
* Fix commit message
* Remove outdated comment in intel_panel.c
* Rename pwm_(min|max) to pwm_level_(min|max)
* Use intel_pwm_get_backlight() in intel_pwm_setup_backlight() instead of
  indirection
* Don't move intel_dp_aux_init_bcklight_funcs() call to bottom of
  intel_panel_init_backlight_funcs() quite yet
v3:
* Reuse intel_panel_bl_funcs() for pwm_funcs
* Explain why we drop lpt_get_backlight()

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Cc: thaytan@noraisin.net
Cc: Vasily Khoruzhick <anarsoul@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114221709.2261452-3-lyude@redhat.com