drm/amdgpu: allocate doorbell index for mes kiq

Allocate a doorbell index for mes kiq queue.

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