drm/amdgpu: accommodate DOMAIN/PL_DOORBELL

This patch adds changes:
- to accommodate the new GEM domain for DOORBELLs
- to accommodate the new TTM PL for DOORBELLs

in order to manage doorbell pages as GEM object.

V2: Addressed reviwe comments from Christian
    - drop the doorbell changes for pinning/unpinning
    - drop the doorbell changes for dma-buf map
    - drop the doorbell changes for sgt
    - no need to handle TTM_PL_FLAG_CONTIGUOUS for doorbell
    - add caching type for doorbell

V3: - Removed unrelated empty line (Christian)
    - Add PL_DOORBELL in mem_type_to_domain() as well (Alex)

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