drm/amdgpu: allocate gart memory when it's required (v3)

Instead of calling gart address space on every bo pin,
allocates it on demand

v2: fix error handling
v3: drop the change on amdgpu_amdkfd_gpuvm.c, not needed.

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>