drm/meson: venc: make drm_display_mode const

Before switching to bridge funcs, make sure drm_display_mode is passed
as const to the venc functions.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304104052.17196-7-narmstrong@baylibre.com