drm/i915: cancel the hangcheck before runtime suspend

The hangcheck function requires the hardware to be working, and if
we're suspending we're going to put the HW in D3 state.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>