drm/i915: don't count cpu ports for fdi B/C lane sharing

This allows us to use all 4 fdi lanes on fdi B when the cpu eDP is
running on pipe C. Yay!

v2: Encapsulate test into a little helper function, as suggested by
Chris Wilson.

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