drm/amdkfd: Separate mqd allocation and initialization

Introduce a new mqd allocation interface and split the original
init_mqd function into two functions: allocate_mqd and init_mqd.
Also renamed uninit_mqd to free_mqd. This is preparation work to
fix a circular lock dependency.

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