[media] media: exynos-gsc: fix build warning

Fixes following build warnings:

gsc-core.c:350:17: warning: 'low_plane' may be used uninitialized
gsc-core.c:371:31: warning: 'high_plane' may be used uninitialized

Reported-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>