i915: fix ironlake edp panel setup (v4)

The eDP spec claims a 20% overhead for the 8:10 encoding scheme used
on the wire. Take this into account when picking the lane/clock speed
for the panel.

v3: some panels are out of spec, try our best to deal with them, don't
refuse modes on eDP panels, and try the largest allowed settings if
all else fails on eDP.
v4: fix stupid typo, forgot to git add before amending.

Fixes several reports in bugzilla:

      https://bugs.freedesktop.org/show_bug.cgi?id=28070

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>