drm/i915: only check for irqs_disabled when disabling LCPLL

Because if we keep the current code, we'll get tons of WARNs on
Broadwell, since the code is Haswell-specific.

We could have also added a Broadwell-specific code there, but it's not
really needed since we never disable LCPLL with the hotplug interrupts
still enabled. So keep the easy-and-simple-to-maintain solution until
we actually need something else.

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