drm/sun4i: Use DRM_GEM_CMA_VMAP_DRIVER_OPS for GEM operations

Our driver makes a typical use of CMA, with GEM object allocated as
GEM CMA objects. Use DRM_GEM_CMA_VMAP_DRIVER_OPS to describe the ops
instead of duplicating them.

Because DRM_GEM_CMA_VMAP_DRIVER_OPS implements a gem_create_object op
which sets per-object funcs (drm_cma_gem_default_funcs), we can also
get rid of free_object_unlocked and gem_vm_ops, which are superseded
by the object funcs.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418130509.3569-1-paul.kocialkowski@bootlin.com