drm/dp: Add documentation for luminance_set

Documentation for luminance_set for struct drm_edp_backlight_info
was missed which causes warnings.

Fixes: 2af612ad ("drm/dp: Introduce new member in drm_backlight_info")
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@gmail.com>
Link: https://lore.kernel.org/r/20250701085054.746408-1-suraj.kandpal@intel.com