drm/exynos: Fix PTN3460 dependency

The following configuration options combination:

CONFIG_DRM_EXYNOS_DP=y
CONFIG_DRM_PTN3460=m

currently leads to the following linker failure:

drivers/built-in.o: In function `exynos_drm_attach_lcd_bridge':
.../drivers/gpu/drm/exynos/exynos_dp_core.c:1004:
undefined reference to `ptn3460_init'

This is because ptn3460_init can't be implemented in a module while
its caller is built into the kernel. So add the proper dependency in
Kconfig so that the above can't happen.

I moved DRM_PTN3460 earlier in Kconfig, next to the I2C helper module
section, so that the user has a chance to select it before moving to
the Exynos-specific section.

IMHO the proper way to solve the problem would be to turn ptn3460 into
a clean I2C driver, similar to the other I2C helper chip drivers. It's
the only way to not sink into impossible-to-guess dependencies. Then
ptn3460 could even be moved together with the other I2C helper chip
drivers.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>