drm/amdgpu: use the whole doorbell space for mes

Use the whole doorbell space for mes. Each queue in one process occupies
one doorbell slot to ring the queue submitting.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>