drm/vc4: hdmi: Add PHY init and disable function

The HDMI PHY in the BCM2711 HDMI controller is significantly more
complicated to setup than in the older BCM283x SoCs.

Let's add hooks to enable and disable the PHY.

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/7216826284dbc60a58bdacd662805d20699e5c80.1599120059.git-series.maxime@cerno.tech