drm/exynos: Use VIDEO_SAMSUNG_EXYNOS_GSC=n as GSC Kconfig dependency

Commit aeefb368 ("drm/exynos: gsc: add device tree support and remove
usage of static mappings") made the DRM_EXYNOS_GSC Kconfig symbol to only
be selectable if the exynos-gsc V4L2 driver isn't enabled, since both use
the same HW IP block.

But added the dependency as depends on !VIDEO_SAMSUNG_EXYNOS_GSC which is
not correct since Kconfig expressions are not boolean but tristate. So it
will only evaluate to 'n' if VIDEO_SAMSUNG_EXYNOS_GSC=y but will evaluate
to 'm' if VIDEO_SAMSUNG_EXYNOS_GSC=m.

This means that both the V4L2 and DRM drivers can be enabled if the former
is enabled as a module, which isn't what we want since otherwise 2 drivers
could attempt to use the hardware at the same time.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>