drm/i915/gvt: only copy the first page for restore inhibit context

if a context is a restore inhibit context, gfx hw only load the first page
for ring context, so we only need to copy from guest the 1 page too.

v3: use "return" instead of "goto" for inhibit case. (zhenyu wang)
v2: move judgement of restore inhibit to a macro in  mmio_context.h

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