drm/amdgpu: add amdgpu_evict_gtt debugfs entry

Allow evicting all BOs from the GTT domain.

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