drm/i915/gvt: move workload destroy out of execlist complete

To let execlist.c only handle execlist handling and keep other
workload cleanup function in scheduler.c to align with other
workload specific handling there. This doesn't change current
code behavior.

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200506094318.105604-1-zhenyuw@linux.intel.com