msm: acpuclock-cortex: Refactor power collapse path

clk_set_rate should not be called during the atomic power collapse path.
Changing the clk source is the only way to change clock rate.

Change-Id: I32117715f452a1c0fb632ad8b3019e924532fe02
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>