drm/vc4: hdmi: Remove vc4_hdmi_encoder

The vc4_hdmi_encoder struct was used exclusively to cache the value
returned by drm_detect_hdmi_monitor() in order to avoid calling it
multiple times.

Now that drm_detect_hdmi_monitor() has been replaced with
drm_display_info.is_hdmi, there is no need to have an extra struct.

Remove vc4_hdmi_encoder.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220420114500.187664-3-jose.exposito89@gmail.com