drm: pin new and unpin old buffer when setting a mode.

This removes the requirement for user space to pin a buffer before
setting a mode that is backed by the pixels from that buffer.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>