drm: shut up invalid edid messages

My cheapo monitor has an invalid block 1, resulting in a lot of dmesg spam every few seconds.

I get it the first time that the entire block is all 0xff..

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: stable@vger.kernel.org [v3.7]
Signed-off-by: Dave Airlie <airlied@redhat.com>