drm/i915/gvt: Add carefully checking in GTT walker paths

When debugging the gtt code, found the intel_vgpu_gma_to_gpa() can
translate any given GMA though the GMA is not valid. This because
the GTT ops suppress the possible errors, which may result in an
invalid PT entry is retrieved by upper caller.

This patch changed the prototype of pte ops to propagate status to
callers. Then we make sure the GTT walker stop as early as when
a error is detected to prevent undefined behavior.

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