drm/amdgpu: fixx NULL pointer deref in gmc_v9_0_get_vm_pte

We not only need to make sure that we have a BO, but also that the BO
has some backing store.

Fixes: d1a372af ("drm/amdgpu: Set MTYPE in PTE based on BO flags")
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>