drm/i915/dp: Allow forcing specific interfaces through enable_dpcd_backlight

Since we now support controlling panel backlights through DPCD using
both the standard VESA interface, and Intel's proprietary HDR backlight
interface, we should allow the user to be able to explicitly choose
between one or the other in the event that we're wrong about panels
reliably reporting support for the Intel HDR interface.

So, this commit adds support for this by introducing two new
enable_dpcd_backlight options: 2 which forces i915 to only probe for the
VESA interface, and 3 which forces i915 to only probe for the Intel
backlight interface (might be useful if we find panels in the wild that
report the VESA interface in their VBT, but actually only support the
Intel backlight interface).

v3:
* Rebase

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/20210114221709.2261452-5-lyude@redhat.com