drm/i915/gvt: Refine virtual reset function

during the emulation of virtual reset:
1. only reset the engine related mmio ending with MMIO
   offset Master_IRQ, not include display stuff.

2. fences are not required to set default
   value as well to prevent screen flicking.

this will fix the issue of Guest screen hang while running
Force tdr in Linux guest.

v2:
- only reset the engine related mmio. (Zhenyu & Zhiyuan)
v3:
- IMR/Ring mode registers are not save/restored. (Changbin)
v4:
- redefine the MMIO reset offset for easy understanding. (Zhenyu)
- pvinfo can be reset. (Zhenyu)
v5:
- add more comments for mmio reset. (Zhenyu)

Cc: Changbin Du <changbin.du@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Lv zhiyuan <zhiyuan.lv@intel.com>
Cc: Zhang Yulei <yulei.zhang@intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>