drm/i915: properly wait for SBI status

Somehow this went unnoticed in the past reviews, but the condition would
never timeout properly.

This was initially introduced in the v2 of original SBI enabling patch.
Highly embarrassing.

Note that we now actually time out for the read, which resulted in gcc
complaining that we can now return unitialized garbage if that
happens. There's not much we can do here because there's not much
point in thread -EIO all the way down through these functions. Hence
simply shut up the compiler.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
[danvet: Added note and squashed uninitialized value shut-up into this
patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>