drm/radeon: Add RADEON_INFO_VA_UNMAP_WORKING query

This tells userspace that it's safe to use the RADEON_VA_UNMAP operation
of the DRM_RADEON_GEM_VA ioctl.

Cc: stable@vger.kernel.org
(NOTE: Backporting this commit requires at least backports of commits
26d4d129,
48afbd70 and
c29c0876

 as well, otherwise using
RADEON_VA_UNMAP runs into trouble)

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>