drm/i915/backlight: Check luminance_set when disabling PWM via AUX VESA backlight

drm/i915/backlight: Check luminance_set when disabling PWM via AUX VESA backlight

When deciding what if PWM funcs need to be disabled take into account
luminance_set too. We do this since it is also used to decide if
we are enabling PWM backlight funcs or not.

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/20260302040613.3324049-5-suraj.kandpal@intel.com