drm/amd/display: Restore correct backlight brightness after a GPU reset

drm/amd/display: Restore correct backlight brightness after a GPU reset

[Why]
GPU reset will attempt to restore cached state, but brightness doesn't
get restored. It will come back at 100% brightness, but userspace thinks
it's the previous value.

[How]
When running resume sequence if GPU is in reset restore brightness
to previous value.

Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 5e19e2b57b6bb640d68dfc7991e1e182922cf867)
Cc: stable@vger.kernel.org