drm/i915: Only call commit_planes when there are things to commit.

The atomic helpers set planes_changed on a crtc_state if there is
any plane_state bound to that crtc. If there's none and there is
no pipe update required the crtc has nothing to update, so vblank
evasion can be skipped.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>