[media] exynos-gsc: Add missing static storage class specifiers

Fixes the following warnings:
drivers/media/platform/exynos-gsc/gsc-core.c:313:5: warning:
symbol 'get_plane_info' was not declared. Should it be static?
drivers/media/platform/exynos-gsc/gsc-core.c:746:28: warning:
symbol 'gsc_ctrl_ops' was not declared. Should it be static?
drivers/media/platform/exynos-gsc/gsc-m2m.c:102:5: warning:
symbol 'gsc_fill_addr' was not declared. Should it be static?
drivers/media/platform/exynos-gsc/gsc-m2m.c:252:16: warning:
symbol 'gsc_m2m_qops' was not declared. Should it be static?

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