drm/radeon: check if BO_VA is set before adding it to the invalidation list

Otherwise we try to clear BO_VAs without an address.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=91141



Signed-off-by: Christian König <christian.koenig@amd.com>
Test-by:  <hadack@gmx.de>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>