drm/vmwgfx: Clean up cursor mobs

Clean up the cursor mob path by moving ownership of the mobs into the
plane_state, and just leaving a cache of unused mobs in the plane
itself.

Signed-off-by: Michael Banack <banackm@vmware.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221022040236.616490-7-zack@kde.org