drm/amdkfd: Use better name to indicate the offset is in dwords

drm/amdkfd: Use better name to indicate the offset is in dwords

The doorbell offset could mean the byte offset or the dword offset,
and the 0 offset place is also different, sometimes the start of PCI
doorbell bar or the start of process doorbell pages. Use better name
to avoid confusion.

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>