drm/i915: Rewrite IVB FDI bifurcation conflict checks

Ignore the current state of the pipe and just check crtc_state->enable
and the number of FDI lanes required. This means we don't accidentally
mistake the FDI lanes as being available of one of the pipes just
happens to be disabled at the time of the check. Also we no longer
consider pipe C to require FDI lanes when it's driving the eDP
transcoder.

We also take the opportunity to make the code a bit nicer looking by
hiding the ugly bits in the new pipe_required_fdi_lanes() function.

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>