uvesafb: Really allow mtrr being 0, as documented and warn()ed

Fixup for commit "uvesafb: Clean up MTRR code"
    (63e28a7a)

Signed-off-by: Sylvain "ythier" Hitier <sylvain.hitier@gmail.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Also-spotted-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>