drm/ttm: allow reserving more than one shared slot v3

Let's support simultaneous submissions to multiple engines.

v2: rename the field to num_shared and fix up all users
v3: rebased

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>