drm/i915: Always normalize return timeout for wait_timeout_ioctl

As we recompute the remaining timeout after waiting, there is a
potential for that timeout to be less than zero and so need sanitizing.
The timeout is always returned to userspace and validated, so we should
always perform the sanitation.

v2 [vsyrjala]: Only normalize the timespec if it's invalid
v3: Add a comment to clarify the situation and remove the now
    useless WARN_ON() (ickle)

Cc: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>