efi_loader: Correctly figure out size for vidconsole

If stdout is vidconsole, we cannot rely on ANSI escape sequences to
query the size, as vidconsole cannot reply on stdin.  Instead special-
case this if stdout is vidconsole.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>