drm/i915/backlight: Take luminance_set into account for VESA backlight

drm/i915/backlight: Take luminance_set into account for VESA backlight

When deciding what functions to enable to help control backlight we
used to only check aux_enable. Now with PANEL_LUMINANCE_OVERRIDE in
picture we need to take care that we do not enable PWM function if
luminance_set is set.

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/15671
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patch.msgid.link/20260302040613.3324049-4-suraj.kandpal@intel.com