drm/i915/gvt: rewrite gt reset handler using new function intel_gvt_reset_vgpu_locked

GT reset and FLR share some operations and they are both implemented in
our new function intel_gvt_reset_vgpu_locked(). This patch rewrite the
gt reset handler using this new function.

Besides, this new implementation fixed the old issue in GT reset. The
old implementation reset GGTT entries which is illegal. We only clear
GGTT entries at PCI level reset.

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