drm: xilinx: gem: Add the Xilinx GEM helper

Add the Xilinx GEM helper, xilinx_drm_gem.c. The file contains
one function to create the dumb GEM object with taking the device
pitch value into consideration. More functions such as own buffer
allocation can be added later on.

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