drm/radeon: fix regression on atom cards with hardcoded EDID record.

Since fafcf94e

 introduced an edid size, it seems to have broken this path.

This manifest as oops on T500 Lenovo laptops with dual graphics primarily.

Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=33812

cc: stable@kernel.org
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>