clk: qcom: add null pointer checks for parent clocks

Add null pointer checks for the parent clock pointers returned
by clk_hw_get_parent() and clk_hw_get_parent_by_index() in
several qcom clock drivers.

Change-Id: I869c9e35b7ec083b34bcc70d18601aec64c3ac97
Signed-off-by: David Collins <collinsd@codeaurora.org>