drm/amdgpu: Fix KFD doorbell SG BO mapping

drm/amdgpu: Fix KFD doorbell SG BO mapping

This change prepares for adding SG BOs that will be used for mapping
doorbells into GPUVM address space.

This type of BO would be mistaken for an invalid userptr BO. Improve
that check to test that it's actually a userptr BO so that SG BOs that
are still in the CPU domain can be validated and mapped correctly.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>