drm/vc4: hdmi: Rename hdmi to vc4_hdmi

The driver isn't consistent with the name given to the vc4_hdmi
structure pointer in its functions. Make sure to use a consistent name.
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/696be840dc427245afe94b43e0b829c728d948a7.1599120059.git-series.maxime@cerno.tech