drm/i915: Fix page fault handler failure

... when wait_seqno() is interrupted by a signal. In this case,
wait_seqno() returns -ERESTARTSYS. Like we already do in drm_ioctl(),
we need to convert this error to a common code such as -EINTR, so the
page fault handler is restarted.

Reported by: Frederic Chardon <chardon.frederic@gmail.com>
Tested by: Frederic Chardon <chardon.frederic@gmail.com>