dma: mmp_pdma: Simplify access to channel drcmr value

As the physical channel and virtual channel point to each other,
pchan->phy->vchan is always equal to pchan. Simplify the code
accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>