drm/i915/dp: Enable Intel's HDR backlight interface (only SDR for now)

So-recently a bunch of laptops on the market have started using DPCD
backlight controls instead of the traditional DDI backlight controls.
Originally we thought we had this handled by adding VESA backlight
control support to i915, but the story ended up being a lot more
complicated then that.

Simply put-there's two main backlight interfaces Intel can see in the
wild. Intel's proprietary HDR backlight interface, and the standard VESA
backlight interface. Note that many panels have been observed to report
support for both backlight interfaces, but testing has shown far more
panels work with the Intel HDR backlight interface at the moment.
Additionally, the VBT appears to be capable of reporting support for the
VESA backlight interface but not the Intel HDR interface which needs to
be probed by setting the right magic OUI.

On top of that however, there's also actually two different variants of
the Intel HDR backlight interface. The first uses the AUX channel for
controlling the brightness of the screen in both SDR and HDR mode, and
the second only uses the AUX channel for setting the brightness level in
HDR mode - relying on PWM for setting the brightness level in SDR mode.

For the time being we've been using EDIDs to maintain a list of quirks
for panels that safely do support the VESA backlight interface. Adding
support for Intel's HDR backlight interface in addition however, should
finally allow us to auto-detect eDP backlight controls properly so long
as we probe like so:

* If the panel's VBT reports VESA backlight support, assume it really
  does support it
* If the panel's VBT reports DDI backlight controls:
  * First probe for Intel's HDR backlight interface
  * If that fails, probe for VESA's backlight interface
  * If that fails, assume no DPCD backlight control
* If the panel's VBT reports any other backlight type: just assume it
  doesn't have DPCD backlight controls

Changes since v4:
* Fix checkpatch issues
Changes since v3:
* Stop using drm_device and use drm_i915_private instead
* Don't forget to return from intel_dp_aux_hdr_get_backlight() if we fail
  to read the current backlight mode from the DPCD
* s/uint8_t/u8/
* Remove unneeded parenthesis in intel_dp_aux_hdr_enable_backlight()
* Use drm_dbg_kms() in intel_dp_aux_init_backlight_funcs()

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/20210114221709.2261452-4-lyude@redhat.com