drm/vc4: hdmi: Remove vc4_hdmi_connector

The vc4_hdmi_connector was only used to switch between drm_connector to
drm_encoder. However, we can now use vc4_hdmi to do the switch, so that
structure is redundant.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/aee5120728db350b19c074de4290eafaf01e6671.1599120059.git-series.maxime@cerno.tech