drm/i915/bios: fix format string of the VBT signature logging

Specify the maximum number of letters to print from the potentially
unterminated buffer, not the minimum. While at it, use sizeof instead of
a magic number.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450271061-32646-2-git-send-email-jani.nikula@intel.com