drm/i915/backlight: Honor VESA eDP backlight luminance control capability

drm/i915/backlight: Honor VESA eDP backlight luminance control capability

The VESA AUX backlight fails to be enable luminance based backlight
mainpulation becaused luminance_set is false by default.
Fix it by using luminance support control capabitliy.

Fixes: e13af5166a359 ("drm/i915/backlight: Use drm helper to initialize edp backlight")
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250823121647.275834-1-aaron.ma@canonical.com
(cherry picked from commit 72136efb875d8438c20b9c8ab72945d474933471)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>