drm/i915: Rename intel_{enable, disable}_plane to intel_{enable, disable}_primary_plane

The new names make it clearer which plane we're talking about.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Resolve small conflict with the haswell_crtc_disable_planes
extraction.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>