drm/i915: remove the IRQs enabled WARN from intel_disable_gt_powersave

This function can be called now with i915 interrupts enabled, so the
corresponding WARN is incorrect, remove it. I think this was spotted by
Paulo during his review, but since I already removed the same WARN
from intel_suspend_gt_powersave() I missed then his point.

Spotted-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>