drm/amdgpu: cleanup MES process level doorbells

MES allocates process level doorbells, but there is no userspace
client to consume it. It was only being used for the MES ring
tests (in kernel), and was written by kernel doorbell write.

The previous patch of this series has changed the MES ring test code to
use kernel level MES doorbells. This patch now cleans up the process level
doorbell allocation code which is not required.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Reviewed-by: Christian König <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>