drm/i915: Take request reference before arming the watchdog timer

Reference needs to be taken before arming the timer. Luckily, given the
default timer period of 20s, the potential to hit the race is extremely
unlikely.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 9b4d0598 ("drm/i915: Request watchdog infrastructure")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326105759.2387104-1-tvrtko.ursulin@linux.intel.com