drm/i915/backlight: Modify function to get VESA brightness in Nits

drm/i915/backlight: Modify function to get VESA brightness in Nits

Modify vesa_get_brightness function to take into account
luminance_control_support and based on that read the appropriate
register and return the value.

--v2
-Changes since we now use luminance instead of nits

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/20250206063253.2827017-6-suraj.kandpal@intel.com