drm: xilinx: Add functions to retrieve the alignment value

The pitch requirement depends on the device, especially from
the DMA engine IP. This function returns the required alignment value
from the DMA engine channel, which can be used for pitch calculation.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>