drm/radeon/kms: memset the allocated framebuffer before using it.

This gets rid of some ugliness, we shuold probably find a way
for the GPU to zero this.

Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/radeon/radeon_fb.c

···
 		goto out_unref;
 	}
 
+	memset_io(fbptr, 0, aligned_size);
+
 	strcpy(info->fix.id, "radeondrmfb");
 	info->fix.type = FB_TYPE_PACKED_PIXELS;
 	info->fix.visual = FB_VISUAL_TRUECOLOR;