drm/i915: dont call irq_put when irq test is on

If test is running, irq_get was not called so we should gain
balance by not doing irq_put

"So the rule is: if you access unlocked values, you use ACCESS_ONCE().
You don't say "but it can't matter". Because you simply don't know."
-- Linus

v2: use local variable so it can't change during test (Chris)

v3: update commit msg and use ACCESS_ONCE (Ville)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>