drm/amdgpu: save/restore backlight level in legacy dce code

Save/restore the backlight level scratch register in S3/S4 so the
backlight level comes back at the previously requested level.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=199047


Fixes: 4ec6ecf4 (drm/amdgpu: drop scratch regs save and restore from S3/S4 handling)
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>