Revert "drm/i915/backlight: Remove try_vesa_interface"

Revert "drm/i915/backlight: Remove try_vesa_interface"

This reverts commit 40d2f5820951dee818d05c14677277048bd85f9f.

Removing the try_vesa_interface gate caused a backlight regression on
panels whose VBT correctly reports INTEL_BACKLIGHT_DISPLAY_DDI and whose
PWM path is the actual backlight control, but whose DPCD optimistically
advertises DP_EDP_BACKLIGHT_AUX_ENABLE_CAP / _BRIGHTNESS_AUX_SET_CAP.
After the commit such panels silently bind to the VESA AUX backlight
funcs; AUX writes complete but the panel ignores them, leaving
brightness stuck (no-op backlight). Observed on at least KBL and TGL
eDP setups.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patch.msgid.link/20260517024709.1016121-1-suraj.kandpal@intel.com
(cherry picked from commit f30fddb4402313aa5301a74d721638d343395269)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>