drm/meson: rename venc_cvbs to encoder_cvbs

Rename the cvbs encoder to match the newly introduced meson_encoder_hdmi.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020123947.2585572-6-narmstrong@baylibre.com