drm/radeon/kms/atom: add support for AdjustDisplayPll

DCE3+ has an AdjustDisplayPll that will adjust the pixel
clock accordingly based on the encoder/transmitter to
handle special hw requirements.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>