protocol: Remove inconsistent line breaks

Enum entries and message arguments are sometimes preceded by a blank line, but
often aren't.

Standardize the format of the protocol specification by removing blank lines
preceding a list of message arguments and enum entries.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>