drm/i915: add asserts for cursor disabled

The cursor is supposed to be disabled during crtc mode set (disabled by
ctrc disable). Assert this is the case.

v2: move cursor disabled assert next to plane asserts (Ville)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>