efifb: Support overriding fields FW tells us with the DMI data.

Some machines apparently give us bogus linelength/stride/pitch data, so
we need to support letting the DMI table override the supplied data.

I bet you can't guess whose machines I'm talking about.

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>