fb: exynos: Fix MIPI/DSI front/back porch settings

The exynos_mipi_dsi_set_main_disp_[hv]porch() functions take front and
back porch arguments in that order. This maps to the fb_videomode
right/lower_margin and left/upper_margin respectively. Fix the caller
accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Donghwa Lee <dh09.lee@samsung.com>