drm: xilinx: fb: Add the Xilinx framebuffer helper

The pitch should be configurable when initializing the fbdev.
This patch set adds and uses the Xilinx framebuffer helper functions
instead of using the drm_fbdev_cma helper functions. In this way,
more low level configuration such as pitch can be handled within
Xilinx DRM driver.

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