This patch:
- adds a doorbell object in kfd pdd structure.
- allocates doorbells for a process while creating its queue.
- frees the doorbells with pdd destroy.
- moves doorbell bitmap init function to kfd_doorbell.c

PS: This patch ensures that we don't break the existing KFD
    functionality, but now KFD userspace library should also
    create doorbell pages as AMDGPU GEM objects using libdrm
    functions in userspace. The reference code for the same
    is available with AMDGPU Usermode queue libdrm MR. Once
    this is done, we will not need to create process doorbells
    in kernel.

V2: - Do not use doorbell wrapper API, use amdgpu_bo_create_kernel
      instead (Alex).
    - Do not use custom doorbell structure, instead use separate
      variables for bo and doorbell_bitmap (Alex)
V3:
   - Do not allocate doorbell page with PDD, delay doorbell process
     page allocation until really needed (Felix)

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