drm/i915/gvt: factor out the scheduler

Factor out the scheduler to a more clear structure, the basic
logic is to find out next vGPU first and then schedule it.
vGPUs were ordered in a LRU list, scheduler scan from the LRU
list head and choose the first vGPU who has pending workload.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>