Use the same code for enabling/disabling planes on all platforms. Rename
the functions to reflect that they're no longer specific to any
platform.

For now we leave the plane enable/disable to ccur at the same old
position in the modeset sequence.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Frob drm_vblank_on conflict.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>