drm/i915/backlight: Add function to change brightness in nits for VESA

drm/i915/backlight: Add function to change brightness in nits for VESA

Create a function that fills in the value for
PANEL_TARGET_LUMINANCE_VALUE which helps in changing the luminance in
nits using VESA interface.

--v2
-Prefer using luminance over nits [Jani]

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/20250206063253.2827017-7-suraj.kandpal@intel.com