drm/i915/backlight: Check Luminance based brightness control for VESA

Check if we are capable of controlling brightness via luminance
which is dependent on PANEL_LUMINANCE_CONTROL_CAPABLE bit being set
on EDP_GENERAL_CAPABILITY_2 register.

--v2
-Prefer using luminance rather than nits [Jani]
-Fix commit message

--v3
-Fix the bit name used in commit message [Arun]
-Use correct edp_dpcd[] to check the capability [Arun]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Tested-by: Ben Kao <ben.kao@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250206063253.2827017-5-suraj.kandpal@intel.com