drm/i915/gvt: Extract mmio_read_from_hw() common function

The mmio read handler for ring timestmap / instdone register are same
as reading hw value directly.

Extract it as common function to reduce code duplications.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>