drm/edid: Fix secondary block fetch.

This makes fetching the second EDID block on HDMI monitors actually
work.  DDC can't transfer more than 128 bytes at a time.  Also,
rearrange the code so the pure DDC bits are separate from block parse.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>