drm/i915: Avoid setting timer->expires to 0

We use timer->expires == 0 to detect if a timer had been cancelled, but
it's a valid expiration we could set. Just skip using 0 and set the
expiry for the next jiffie.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403073657.13427-1-chris@chris-wilson.co.uk