drm/vc4: hdmi: Remove vc4_dev hdmi pointer

Now that we don't have any users anymore, we can kill that pointer.

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/d7ebf50319bbd902e4b7b7c39ae1612c3b38e6b2.1599120059.git-series.maxime@cerno.tech