configs/olimex_a20_olinuxino: fix mali not working because Linux version >= 4.20

Since on olinuxino_lime/lime2 we have a Linux version >= 4.20 we need to
pass drm_kms_helper.drm_leak_fbdev_smem=1 to kernel command-line that
really enables DRM_FBDEV_LEAK_PHYS_SMEM. CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM
only adds the code for this, but drm_leak_fbdev_smem is 0 by default, so we
need to override it with 1. Same goes for drm_fbdev_overalloc that
must be at least 200 for having a double buffer that is required by
Mali. This fixes both olimex_a20_olinuxino_lime and lime2 that use
extlinux.conf.

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

board/olimex/a20_olinuxino/rootfs_overlay/boot/extlinux/extlinux.conf

@@ -1,4 +1,4 @@
 label Olinuxino
     kernel /boot/zImage
     fdtdir /boot
-    append console=ttyS0,115200 root=/dev/mmcblk0p1 rootwait panic=10 ${extra}
+    append console=ttyS0,115200 root=/dev/mmcblk0p1 rootwait panic=10 drm_kms_helper.drm_leak_fbdev_smem=1 drm_kms_helper.drm_fbdev_overalloc=200 ${extra}