drm/vc4: hdmi: do not return negative values from .get_modes()

The .get_modes() hooks aren't supposed to return negative error
codes. Return 0 for no modes, whatever the reason.

Cc: Maxime Ripard <mripard@kernel.org>
Cc: stable@vger.kernel.org
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/dcda6d4003e2c6192987916b35c7304732800e08.1709913674.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>