[media] s5p-g2d: Add missing static storage class in g2d.c file

Fixes the following sparse warnings:
drivers/media/video/s5p-g2d/g2d.c:68:18: warning: symbol 'def_frame' was not declared. Should it be static?
drivers/media/video/s5p-g2d/g2d.c:80:16: warning: symbol 'find_fmt' was not declared. Should it be static?
drivers/media/video/s5p-g2d/g2d.c:205:5: warning: symbol 'g2d_setup_ctrls' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>