drm/amdkfd: Renaming dqm->packets to dqm->packet_mgr

Renaming packets to packet_mgr to reflect the real meaning
of this variable.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Christian Konig <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>