drm/radeon: fix UVD message buffer validation

commit 112a6d0c071808f6d48354fc8834a574e5dcefc0 upstream.

When the message buffer is currently moving block until it is idle again.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>