drm/amdgpu: fix check in gmc_v9_0_get_vm_pte()

The coherency flags can only be determined when the BO is locked and that
in turn is only guaranteed when the mapping is validated.

Fix the check, move the resource check into the function and add an assert
that the BO is locked.

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