drm/i915/gvt: move write protect handler out of mmio emulation function

It's a bit confusing that page write protect handler is live in
mmio emulation handler. This moves it to stand alone gvt ops.

Also remove unnecessary check of write protected page access
in mmio read handler and cleanup handling of failsafe case.

v2: rebase

Reviewed-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>