drm/i915: don't assert_pch_ports_disabled on LPT

That function is made for IBX. Running it on LPT will trigger tons of
"unclaimed register" errors. The only port remaining on LPT is
PCH_ADPA.

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