dmaengine: dw: balance PM runtime calls

In case of PCI driver we will get a warning:
	dw_dmac_pci 0000:00:18.0: Unbalanced pm_runtime_enable!
	dw_dmac_pci 0000:00:18.0: DesignWare DMA Controller, 8 channels

This happens due to pm_runtime_enable() call from the driver when PM runtime is
enabled by core.

This patch moves that call to the platform driver where it might make sense.

Fixes: bb32baf7

 (dmaengine: dw: enable runtime PM)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>