drm/i915/gvt: fix crash in vgpu_reset_execlist

We initiate vgpu->workload_q_head via for_each_engine
macro which may skip unavailable engines. So we should
follow this rule anywhere. The function
intel_vgpu_reset_execlist is not aware of this. Kernel
crash when touch a uninitiated vgpu->workload_q_head[x].
Let's fix it by using for_each_engine_masked and skip
unavailable engine ID. Meanwhile rename ring_bitmap to
general name engine_mask.

v2: remove unnecessary engine activation check (zhenyu)

Signed-off-by: Du, Changbin <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>