video: fb_defio.c: local functions should not be exposed globally

Functions not referenced outside of a source file should be marked
static to prevent them from being exposed globally.

Quiets the sparse warning:

warning: symbol 'fb_deferred_io_page' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Jaya Kumar <jayalk@intworks.biz>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>