drm/vc4: hdmi: Use debugfs private field

We're calling vc4_debugfs_add_file with our struct vc4_hdmi pointer set
in the private field, but we don't use that field and go through the
main struct vc4_dev to get it.

Let's use the private field directly, that will save us some trouble
later on.

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/24028dc06c379dbc71f98e027cce2839fdd446ce.1599120059.git-series.maxime@cerno.tech