drm/edid: Try harder to fix up base EDID blocks

Requiring the first byte of the EDID base block header to be 0 means we
don't fix up as many transfer errors as we could.  Instead have the
callers specify whether it's meant to be block 0 or not, and
conditionally run header fixup based on that.

Bugzilla: https://bugzilla.redhat.com/812890
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>