drm/i915/gvt: refactor function intel_vgpu_submit_execlist

drm/i915/gvt: refactor function intel_vgpu_submit_execlist

The function intel_vgpu_submit_execlist could be more simpler. It
actually does:
  1) validate the submission. The first context must be valid,
     and all two must be privilege_access.
  2) submit valid contexts. The first one need emulate schedule_in.

We do not need a bitmap, valid desc copy valid_desc. Local variable
emulate_schedule_in also can be optimized out.

v2: dump desc content in err msg (Zhi Wang)

Signed-off-by: Changbin Du <changbin.du@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>