drm/amdgpu: remove CONFIG_DRM_AMDGPU_GART_DEBUGFS

Removed unused CONFIG_DRM_AMDGPU_GART_DEBUGFS code.
We can use umr instead of this gart debugfs.

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