drm/i915/gt: Clear the execlists timers upon reset

Across a reset, we stop the engine but not the timers. This leaves a
window where the timers have inconsistent state with the engine, but
should only result in a spurious timeout. As we cancel the outstanding
events, also cancel their timers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204151234.19729-4-chris@chris-wilson.co.uk