msm: dma: Use clk_prepare and clk_unprepare

clk_prepare and clk_unprepare are now required to be called before
clk_enable and after clk_disable. Do this for the dma driver.
Additionally, split the msm_dmov_clk_toggle function into two, since we
always know the value of the second argument at compile time.

Change-Id: Ib6904c551d7d76badadd47e63c3b7f1cdf51ad33
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>