dmaengine: tegra-apb: Keep clock enabled only during of DMA transfer

dmaengine: tegra-apb: Keep clock enabled only during of DMA transfer

It's a bit impractical to enable hardware's clock at the time of DMA
channel's allocation because most of DMA client drivers allocate DMA
channel at the time of the driver's probing, and thus, DMA clock is kept
always-enabled in practice, defeating the whole purpose of runtime PM.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://lore.kernel.org/r/20200209163356.6439-13-digetx@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>