drm/amdgpu: revert "cleanup setting bulk_movable"

This reverts commit 8466cc61.

It can trigger a reference counter bug in TTM. Need to investigate further, but
for now revert the offending change.

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