drm/i915: Only call mod_timer() if not already pending

The final arrangement of updating timer->expires and calling mod_timer()
used in

commit 672e7b7c
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Nov 19 09:47:19 2014 +0000

    drm/i915: Don't continually defer the hangcheck

turns out to be very unsafe. Try again.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>