drm/i915: Convert EFAULT into a silent SIGBUS

EFAULT will be a possible return code where backing storage is
transient, such after it is purged by madvise. As such it is to be
expected and so should not trigger a WARN inside i915_gem_fault() but be
converted silently to SIGBUS.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>