drm/i915: suppress false PLL state warnings on non-GMCH platforms

The checks for PLL enabled state on CPU ports are valid only on GMCH
platforms but atm we'd also call them on non-PCH-split/non-GMCH
platforms like BXT, triggering false warnings. Until the proper check is
implented for these platforms simply disable the check.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>