Similar to the hsw/bdw enable sequence rewrite.

v3: replace vblank wait with a comment

v4: expand the comment on what should be done with the vblank wait
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>