drm/i915: Handle ERESTARTSYS during page fault

During a page fault and rebinding the buffer there exists a window for a
signal to arrive during the i915_wait_request() and trigger a
ERESTARTSYS. This used to be handled by returning SIGBUS and thereby
killing the application. Try 'cairo-perf-trace & cairo-test-suite' and
watch X go boom!

The solution as suggested by H. Peter Anvin is to simply return NOPAGE and
leave the higher layers to spot we did not fill the page and resubmit
the page fault.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
[anholt: Mostly squash it with another commit]