drm/i915/gvt: Refine the snapshort range of I915 MCHBAR to optimize gvt-g boot time

drm/i915/gvt: Refine the snapshort range of I915 MCHBAR to optimize gvt-g boot time

Currently it will take the snapshot of the MCHBAR registers for gvt-g
initialization so that it can be used for guest vgpu. And it will cover
from 0x140000 to 0x17ffff. In fact based on the HW spec most of them are
meanlingless and some time is wasted to read these register.
Only the range of 0x144000 to 0x147fff contains the valid definition.
So the range of capturing I915 MCHBAR register is refined, which helps
to optimize the gvt-g boot time.

V1->V2: Move the register definition into reg.h

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>