drm/i915/gvt: Legacy HSW related MMIO handler clean up

drm/i915/gvt: Legacy HSW related MMIO handler clean up

remove all the legacy pre-BDW mmio handlers and the corresponding
usage/definition since pre-BDW platforms are not supported in GVT
environment.

v2:
- clean up all the left dirty code before BDW, e.g
  all D_HSW usage and itself, D_IVB, D_PRE_BDW. (Zhenyu)
v3:
- change is based on gvt-staging. (Zhenyu)

Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>