drm/amdgpu: use helper in amdgpu_gart_unbind

When amdgpu_gart_invalidate_tlb helper is introduced this part was left
out of the conversion. Avoid the code duplication here.

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