drm/i915: release cursor when crtc is destroyed

crtc is holding a reference to a cursor bo and it needs
to be released when crtc is destroyed so that we don't leak
the cursor bo.

v2: Enhance set and move cursor so that disabled
cursor is handled correctly (Ville Syrjälä)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>