drm/amdgpu: reserve/unreserve vmid by vm ioctl v4

add reserve/unreserve vmid funtions. Used to reserve
vmids for certain shader debugging functionality that
required a fixed vmid for the life of the debug.

v3:
only reserve vmid from gfxhub
v4:
fix racy condition

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>