drm/i915/gvt: set default value to 0 for unhandled mmio regs

for a handled mmio reg,  its default value is read from hardware, while
for an unhandled mmio regs,  its default value would be random if not
explicitly set to 0

Signed-off-by: Zhao Yan <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>