drm/radeon: add a way to get and set initial buffer domains v2

When passing buffers between processes, the receiving process needs to know
the original buffer domain, so that it doesn't accidentally move the buffer.

v2: reserve the buffer

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>