drm/vc4: hdmi: rework connectors and encoders

the vc4_hdmi driver has some custom structures to hold the data it needs to
associate with the drm_encoder and drm_connector structures.

However, it allocates them separately from the vc4_hdmi structure which
makes it more complicated than it needs to be.

Move those structures to be contained by vc4_hdmi and update the code
accordingly.

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: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/93b418d63c876355af2b3d3afebe31a256268623.1599120059.git-series.maxime@cerno.tech