EXYNOS: Add dummy definition to fix compilation dependency on CONFIG_EXYNOS_MIPI_DSIM

When only DP is used, we need not enable CONFIG_EXYNOS_MIPI_DSIM.
But if we do not select CONFIG_EXYNOS_MIPI_DSIM, exynos_fb.c throws
error saying exynos_mipi_dsi_init() not defined. So, we add
dummy definition for exynos_mipi_dsi_init when CONFIG_EXYNOS_MIPI_DSIM
is not defined.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Donghwa Lee <dh09.lee@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>