drm/amdkfd: Separate doorbell allocation from PASID

PASID management is moving into KGD. Limiting the PASID range to the
number of doorbell pages is no longer practical.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>