package/sunxi-mali-mainline-driver: update help section to make it work on Linux >= 4.20

On Linux version >= 4.20 in order to have mali working we need to pass
drm_kms_helper.drm_leak_fbdev_smem=1 and at least
drm_kms_helper.drm_fbdev_overalloc=200 to have a double buffer fbdev.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/sunxi-mali-mainline-driver/Config.in

@@ -26,4 +26,14 @@ config BR2_PACKAGE_SUNXI_MALI_MAINLINE_DRIVER
 	  enabled:
 	  - CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM
 
+	  The following kernel command line options must be passed:
+
+	  - drm_kms_helper.drm_leak_fbdev_smem = 1
+	    since drm_kms_helper.drm_leak_fbdev_smem is set to 0 by
+	    default
+
+	  - drm_kms_helper.drm_fbdev_overalloc >= 200
+	    since the driver requires at least double buffering and
+	    fails to work with the default value(100).
+
 	  https://github.com/mripard/sunxi-mali/blob/master/README.md