i915: signedness bug in check_overlay_src()

"depth" should be signed in case packed_depth_bytes() returns -EINVAL.

This probably doesn't make a difference at runtime.  In the original
code we would return -EINVAL later if (rec->offset_Y % 4294967274) is
non-zero.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>