drm/meson: Fix CVBS initialization when HDMI is configured by bootloader

When the HDMI output is configured by the bootloader, there is mismatch is the
pipeline configuration and the Vsync interrupt fails to trigger.

This commit disables the HDMI blocks in the probe phase.

Fixes: bbbe775e ("drm: Add support for Amlogic Meson Graphic Controller")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>