drm/i915: Don't try to disable plane if it's already disabled

Check plane->fb in intel_disable_plane() to determine if the plane
is already disabled.

If the plane has an fb, then it must also have a crtc, so we can drop
the plane->crtc check and just call intel_enable_primary() directly.

v2: WARN and bail if the plane doesn't have a crtc when it should

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