drm/i915: be more strict about HAS_PCH_NOP() usage

HAS_PCH_NOP() implies a PCH platform without south display, not generic
disabled display. Prefer num_pipes == 0 for PCH independent checks.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608123330.31003-5-jani.nikula@intel.com