drm/radeon: restore backlight level on resume

Restore the backlight level on resume.  Some systems
need to explicitly restore the backlight level on
resume.

Fixes panel resume on my Trinity laptop and may fix the
following bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=43829
https://bugzilla.kernel.org/show_bug.cgi?id=46241

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>