drm: exynos: dsi: Add mode_set function

Get the display mode settings via mode_set bridge function
instead of explicitly de-reference.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211122070633.89219-3-jagan@amarulasolutions.com