drm/amdgpu: use doorbell mgr for kfd kernel doorbells

This patch:
- adds a doorbell bo in kfd device structure.
- creates doorbell page for kfd kernel usages.
- updates the get_kernel_doorbell and free_kernel_doorbell functions
  accordingly

V2: Do not use wrapper API, use direct amdgpu_create_kernel(Alex)
V3:
 - Move single variable declaration below (Christian)
 - Add a to-do item to reuse the KGD kernel level doorbells for
   KFD for non-MES cases, instead of reserving one page (Felix)

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