drm/meson: venc: dmt mode must use encp

The video mode for DMT is only populated to support encp.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez.ortiz@gmail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1542048069-22603-1-git-send-email-jramirez@baylibre.com