staging: tidspbridge: remove dmm_init() and dmm_exit()

The dmm module has a dmm_init() and a dmm_exit() whose only purpose is
to keep a reference counting which is not used at all.

This patch removes these functions and the reference count variable.

There is no functional changes.

Signed-off-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>