clk: msm: clock: Refactor the aggr_sibling_rates logic

Return early if the branch clock does not need the rate
aggregation support. Only take the common branch_clk_lock
when needed; doing so unnecessarily will lead to lock
contention in some cases.

CRs-Fixed: 972426
Change-Id: I00476d1c193df025a1da19aa0d0ddf0f0462b4d0
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>