drm/i915/gvt: refine function emulate_mmio_read/write

These 2 functions are coded by multiple person in multiple patches. The
'return' and 'goto err' are mix-used in same place, which cause the
function looks disorder. Unify to use only 'goto' so that the gvt lock
is acquired in one place and released in one place.

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>