drm/exynos: Use VIDEO_SAMSUNG_S5P_G2D=n as G2D Kconfig dependency

Commit 254d4d11 ("drm/exynos: Add dependency for G2D in Kconfig") made
the DRM_EXYNOS_G2D symbol to only be selectable if the s5p-g2d V4L2 driver
is not enabled, since both use the same HW IP block.

But added the dependency as depends on !VIDEO_SAMSUNG_S5P_G2D which isn't
correct since Kconfig expressions are not boolean but tristate. So it will
only evaluate to 'n' if VIDEO_SAMSUNG_S5P_G2D=y but it will evaluate to m
if VIDEO_SAMSUNG_S5P_G2D=m.

This means that both the V4L2 and DRM drivers can be enabled if the former
is enabled as a module, which is not what we want.

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