OMAP clockdomain: if no autodeps exist, don't try to add or remove them

_clkdm_add_autodeps() and _clkdm_del_autodeps() will attempt to dereference
a NULL pointer if no autodeps were supplied to clkdm_init().

Based on a patch from Roel Kluin <roel.kluin@gmail.com> - thanks Roel.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Roel Kluin <roel.kluin@gmail.com>