drm/prime: support to cache mapping

The drm prime also can support it like GEM CMA supports to cache
mapping. It doesn't allow multiple mappings for one attachment.

[airlied: rebased on top of other prime changes]
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>