drm/amdkfd: Fix SDMA in CPX mode

When creating a user-mode SDMA queue, CP FW expects
driver to use/set virtual SDMA engine id in MAP_QUEUES
packet instead of using the physical SDMA engine id.
Each partition node's virtual SDMA number should start
from 0. However, when allocating doorbell for the queue,
KFD needs to allocate the doorbell from doorbell space
corresponding to the physical SDMA engine id, otherwise
the hwardware will not see the doorbell press.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Amber Lin <Amber.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>