drm/amdkfd: Avoid using doorbell_off as offset in process doorbell pages

dorbell_off in the queue properties is mainly used for the doorbell dw
offset in pci bar. We should not set it to the doorbell byte offset in
process doorbell pages. This makes the code much easier to read.

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