qcom: clock-generic: Check if a parent is already at a requested rate

If it is guaranteed that taking the prepare lock of a
clock ensures that the rate of downstream sources cannot
change, a mux between that clock and those downstream sources
can safely check the rate of those sources using clk_get_rate
and use clk_round_rate to request new rates.

Sometimes, the first source on a mux may be able to support a
requested rate, but a subsequent source may already be at that
rate. Use clk_get_rate instead of clk_round_rate to find the
latter source instead of reprogramming the former.

Change-Id: I950a04e005c413559b392d847ef3dff4f7d61ef5
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>