drm/i915: Don't try to enable cursor from setplane when crtc is disabled

Make sure the cursor gets fully clipped when enabling it on a disabled
crtc via setplane. This will prevent the lower level code from
attempting to enable the cursor in hardware.

Cc: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>