drm/i915: fix FDI frequency check

Since mode->clock is in kHz we should be checking against 2700000
instead of just 27000.  This patch gets my x201s working again (well
working as well as it ever was anyway).

When looking for this I also noticed we set link_bw to 270000, but the
calculation is different.  Does it also need to use kHz or we using
10kHz internally?

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>