drm exynos: use drm_fb_helper_set_par directly

info->fix.visual already is correctly set from drm_fb_helper_fill_fix.
info->fix.line_length is also set from drm_fb_helper_fill_fix,
so drm_fb_helper_set_par directly instead of a custom
exynos_drm_fbdev_set_par.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>