And cache the value.  These can be used by the backlight callbacks
and modesetting functions.

v2: rebase on latest backlight changes.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1337Reviewed-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>