commit 4cee6a90 upstream.

So that the bl encoder will be null if the GPU does not
control the backlight.
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[ luis: backported to 3.16:
  - change radeon_link_encoder_connector() instead of
    radeon_encoder_add_backlight() ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>