[media] cec-funcs.h: add missing vendor-specific messages

The cec-funcs.h header was missing support for these three vendor-specific messages:

CEC_MSG_VENDOR_COMMAND
CEC_MSG_VENDOR_COMMAND_WITH_ID
CEC_MSG_VENDOR_REMOTE_BUTTON_DOWN

Add wrappers for these messages.

I originally postponed adding these wrappers due to the fact that the argument is
just a byte array which cec-ctl couldn't handle at the time, and then I just forgot
to add them once the CEC framework was finalized.

It wasn't until an attempt to transmit a vendor specific command was made that I
realized that these wrappers were missing.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>