efifb: exit if framebuffer address is invalid

efifb will attempt to ioremap a framebuffer even if its starting address
is 0, failing and causing an ugly backtrace in the process.  Exit before
probing if this is the case.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Peter Jones <pjones@redhat.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>