clk: qcom: Cache vdd vote if clk is unprepared

When set_rate is called on unprepared clocks new vdd_level
requirement is not cached, which leads to incorrect votes
when later clk_prepare is called. Clocks may not be prepared
when clk pre change rate is called for set_rate change request,
thus cache vdd level and only apply vote during clk prepare.

Change-Id: I54d9a3c75034cd9e67e683c3f71088f2c7cd3db1
Signed-off-by: Vivek Aknurwar <viveka@codeaurora.org>