drm/i915/gvt: remove scheduler_mutex in per-engine workload_thread

For the vGPU workloads, now GVT-g use per vGPU scheduler, the per-ring
work_thread only pick workload belongs to the current vGPU. And with time
slice based scheduler, it waits all the engines become idle before do vGPU
switch. So we can run free dispatch in per-ring work_thread, different ring
running in different 'vGPU' won't happen.

For the workloads between vGPU and Host, this scheduler_mutex can't block
host to dispatch workload into other ring engines.

Here remove this mutex since it impacts the performance when applications
use more than 1 ring engines in 1 vgpu.

ring0 running in vGPU1, ring1 running in Host. Will happen.
ring0 running in vGPU1, ring1 running in vGPU2. Won't happen.

Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>