drm/amdgpu: fix userqueue UAPI comments

This patch fixes some of the pending UAPI review comments
from the libDRM/UAPI review process.

- It updates some outdated comments in the userqueue UAPI header
  highlighted during the libdrm UAPI review.
- It removes the GDS BO support which was found unused.
- It also removes the unused flags parameter from the UAPI.
- It also adds a padding variables in userqueue in/out structures.

(Pierre-Eric and Marek)
  - clarify comments on top of drm_amdgpu_userq_in
  - clarify comment for queue_id (in)
  - clarify comment for mqd
  - clarify comment for compute MQD size
  - clarify comment for queue_id (out)
  - remove GDB object from BO object list
  - remove the unused flags parameter

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