drm/i915/gvt: Support event based scheduling

This patch decouple the time slice calculation and scheduler, let
other event be able to trigger scheduling without impact the
calculation for QoS.

v2: add only one new enum definition.
v3: fix typo.

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