drm/i915/gvt: Make the MMIO attribute wrappers be inline

Function calls are expensive. I have see obvious overhead call to
these wrappers in perf data, especially from the cmd parser side.
So make these simple wrappers be inline to kill them all.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>