drm/i915: Check hw state in assert_can_disable_lcpll

All the other checks also check hw state, so checking our software
refcounts for the plls looks a bit odd. Also this will simplify the
conversion over to the shared dpll framework, which itself has massive
amounts of checks to make sure that we never leave a display pll
enabled when we shouldn't.

So after that conversion we should stil have a good enough coverage of
asserts for entering pc8/runtime pm on hsw/bdw.

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