drm/amdgpu: Restore LCD backlight level on resume

Instead of only enabling the backlight (which seems to set it to max
brightness), just re-set the current backlight level, which also takes
care of enabling the backlight if necessary.

Port of radeon commit:
drm/radeon: Restore LCD backlight level on resume (>= R5xx)

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org