drm/amdgpu: add helpers to create userqueue object

drm/amdgpu: add helpers to create userqueue object

This patch introduces amdgpu_userqueue_object and its helper
functions to creates and destroy this object. The helper
functions creates/destroys a base amdgpu_bo, kmap/unmap it and
save the respective GPU and CPU addresses in the encapsulating
userqueue object.

These helpers will be used to create/destroy userqueue MQD, WPTR
and FW areas.

V7:
- Forked out this new patch from V11-gfx-userqueue patch to prevent
  that patch from growing very big.
- Using amdgpu_bo_create instead of amdgpu_bo_create_kernel in prep
  for eviction fences (Christian)

V9:
 - Rebase
V10:
 - Added Alex's R-B

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: Alex Deucher <alexander.deucher@amd.com>