drm: xlnx: mixer: logo layer fix

drm: xlnx: mixer: logo layer fix

This patch fixes the occasional kernel oops when programing logo layer.
Issue was because of accessing/programing wrong memory.
Following fixes corrected this calculation :
- using the currect buffer b_buf instead of r_buf
- correcting the register address calculation by one offset
- corrected pixel_alpha selection based on color format logic

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