x86-64, setup: Inhibit decompressor output if video info is invalid

Inhibit output from the kernel decompressor if the video information
is invalid.  This was already the case for 32 bits, make 64 bits
match.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <tip-*@git.kernel.org>