drm/i915: Account for sink max TMDS clock when checking the port clock

It's perfectly legal for the sink to support 12bpc only for
some lower resolution modes, while the higher resolution modes
can only be used with 8bpc. So let's take the sink's max TMDS clock
into account before we go and decide that a particular mode can
be used with 12bpc.

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>
Link: http://patchwork.freedesktop.org/patch/msgid/1475070703-6435-11-git-send-email-ville.syrjala@linux.intel.com