drm/radeon: remove radeon_bo_clear_va

Won't work anyway, instead WARN_ON if the VA list isn't
empty when we free the BO.

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