drm/radeon: fix adding all VAs to the freed list on remove v2

We only should do so when the BO_VA was actually mapped.
Otherwise we get a nice error message on the next CS.

v2: It actually doesn't matter if it was invalidated or not,
    if it was mapped we need to clear the area where it was mapped.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>