drm/amdgpu/display: add helper functions to get/set backlight (v2)

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/1337


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>