msm: clock-local2: Fix rcg_clk_set_rate race condition

Setting the parent corresponding to the new source
for an RCG clock while not holding the per-clock
spinlock for that RCG may cause race conditions.
Fix this. Also, use the new __clk_pre_reparent and
__clk_post_reparent APIs to switch the RCG source.

Change-Id: Idac6c006630cc617a48e7126e281cd78aac87fef
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>