EXYNOS5: Make all display related code dependent on CONFIG_LCD

EXYNOS5: Make all display related code dependent on CONFIG_LCD

u-boot compilation fails for smdk5250 when we deselect CONFIG_LCD
from the main config file.

Following error was observed:
drivers/video/libvideo.o: In function `exynos_lcd_init':
/home/ajay/u-boot-samsung/drivers/video/exynos_fb.c:68: undefined reference to `lcd_set_flush_dcache'

This is because exynos video drivers have dependency on CONFIG_LCD.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>