msm: clock: Decrease voltage voting overhead for RPM and PCOM clocks

The clk_get_rate() calls that were added to  clk_enable(), clk_disable(),
and clk_set_rate() to support voltage scaling result in unnecessary
round-trips to the remote processor for remotely-managed clocks.

Fix this by caching the last set rate in 'struct clk' instead of calling
clk_get() to query it.

Change-Id: I7c303afe3988c3731efb60d1067842cf586baf7e
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>