drm/i915: Avoid refcount_inc on known zero count

In intel_wakeref_auto, we use refcount_inc_not_zero to detect the first
use and initialise the timer. On doing so, we have to avoid using
refcount_inc on that zero count as the debug code flags that as an
error:
	refcount_t: increment on 0; use-after-free.

Rearrange the code so that if we know the count is 0 and we are
initialising, we explicitly set it to 1.

Fixes: b27e35ae ("drm/i915: Keep user GGTT alive for a minimum of 250ms")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528154053.22004-1-chris@chris-wilson.co.uk