video: exynos: Fix S6E8AX0 LCD driver build error

Enable S6E8AX0 LCD driver only if LCD_CLASS_DEVICE is a built-in driver.
Else we get the following errors due to missing symbols:
drivers/built-in.o: In function `s6e8ax0_probe':
:(.text+0x51aec): undefined reference to `lcd_device_register'
:(.text+0x51c44): undefined reference to `lcd_device_unregister'

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>