drm/i915: Remove the SPLL==270Mhz assumption from intel_fdi_link_freq()

drm/i915: Remove the SPLL==270Mhz assumption from intel_fdi_link_freq()

Instead of assuming we've correctly set up SPLL to run at 270Mhz for
FDI, let's use the port_clock from pipe_config which should be what
we want. This would catch problems if someone misconfigures SPLL for
whatever reason.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455738073-14502-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>