drm/vc4: Bring HDMI up from power off if necessary.

If the firmware hadn't brought up HDMI for us, we need to do its
power-on reset sequence (reset HD and and clear its STANDBY bits,
reset HDMI, and leave the PHY disabled).

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 851479ad)