drm/i915: Kill IRONLAKE_FDI_FREQ check

ironlake_fdi_compute_config() already checks that we have enough
FDI bandwidth. And it doesn't just use a hardcoded value but takes
into account factors such as the actual FDI frequency, shared FDI
B/C lanes, etc.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>