drm: Set cliprects to NULL when changing drawable to having 0 cliprects.

This avoids setting the cliprects pointer to a zero-sized allocation.

Signed-off-by: Zhenyu Wang <zhenyu.z.wang@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>