drm/amdgpu: add usermode queue base code

This patch adds IP independent skeleton code for amdgpu
usermode queue. It contains:
- A new files with init functions of usermode queues.
- A queue context manager in driver private data.

V1: Worked on design review comments from RFC patch series:
(https://patchwork.freedesktop.org/series/112214/

)
- Alex: Keep a list of queues, instead of single queue per process.
- Christian: Use the queue manager instead of global ptrs,
           Don't keep the queue structure in amdgpu_ctx

V2:
 - Reformatted code, split the big patch into two

V3:
- Integration with doorbell manager

V4:
- Align the structure member names to the largest member's column
  (Luben)
- Added SPDX license (Luben)

V5:
- Do not add amdgpu.h in amdgpu_userqueue.h (Christian).
- Move struct amdgpu_userq_mgr into amdgpu_userqueue.h (Christian).

V6: Rebase
V9: Rebase
V10: Rebase + Alex's R-B

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>