drm/amdgpu: add amdgpu_evict_vram debugfs file

Torture test for MM and VM support, can be used to evict all VRAM while
the system is under load.

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