drm/i915: put runtime PM only when we actually release force_wake

When we call gen6_gt_force_wake_put we don't actually put force_wake,
we just schedule gen6_force_wake_work through mod_delayed_work, and
that will eventually release force_wake.

The problem is that we call intel_runtime_pm_put directly at
gen6_gt_force_wake_put, so most of the times we put our runtime PM
reference before the delayed work happens, so we may runtime suspend
while force_wake is still supposed to be enabled if the graphics
autosuspend_delay_ms is too small.

Now the nice thing about the current code is that after it triggers
the delayed work function it gets a refcount, and it only triggers the
delayed work function if refcount is zero. This guarantees that when
we schedule the funciton, it will run before we try to schedule it
again, which simplifies the problem and allows for the current
solution to work properly (hopefully!).

v2: - Keep the VLV refcounts balanced (Jesse)

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>