Rename HDMI ALSA device names, check for enable state

HDMI Alsa devices renamed to match names used by DRM, to
HDMI 1 and HDMI 2

Check for which HDMI devices are connected and only create
devices for those that are present.

The rename of the devices might cause some backwards compatibility
issues, but since this particular part of the driver needs to be
specifically enabled, I suspect the number of people who will see
the problem will be very small.

Signed-off-by: James Hughes <james.hughes@raspberrypi.org>