video: exynos: fix modular build

The s6e8ax0 driver has a dependency on BACKLIGHT_CLASS_DEVICE,
which can be configured as a loadable module, so we have to
make the driver a tristate symbol as well, to avoid this error:

drivers/built-in.o: In function `s6e8ax0_probe':
:(.text+0x23a48): undefined reference to `devm_backlight_device_register'

This also means we get another error from a missing export, which
this fixes as well:

ERROR: "exynos_mipi_dsi_register_lcd_driver" [drivers/video/fbdev/exynos/s6e8ax0.ko] undefined!

The drivers are all written to be loadable modules already,
except the Kconfig options for that are missing, which makes
the patch really easy.

Finally, the EXYNOS_VIDEO option is turned into tristate as well
for good measure, as all framebuffer drivers should be configurable
as modules, though this change is not strictly necessary.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>