drm/amdgpu: use doorbell mgr for MES kernel doorbells

This patch:
- Removes the existing doorbell management code, and its variables
  from the doorbell_init function, it will be done in doorbell
  manager now.
- uses the doorbell page created for MES kernel level needs (doorbells
  for MES self tests)
- current MES code was allocating MES doorbells in MES process context,
  but those were getting written using kernel doorbell calls. This patch
  instead allocates a MES kernel doorbell for this (in add_hw_queue).

V2: Create an extra page of doorbells for MES during kernel doorbell
    creation (Alex)
V4: Move MES doorbell size and page offset objects in this patch from
    patch 6.

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: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Arvind Yadav <arvind.yadav@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>