drm/amdkfd: Rename read_doorbell_id in MQD functions

Rename read_doorbell_id function to a more meaningful name,
implying what it is used for. No functional change.

Suggested-by: Jay Cornwall <jay.cornwall@amd.com>
Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>