oniro-linux.conf: Add rPi4 specific GPU settings

Weston failed to start on rPi4 with 7' HDMI display due to lack of dri
card. Switching to vc4-kms-v3d driver fixes the problem. GPU mem size is set to
be 128 Mb because this configuration has been tested on vending machine
blueprint thus copying it here for consistency.

Closes: https://booting.oniroproject.org/distro/oniro/-/issues/182
Related: https://github.com/agherzan/meta-raspberrypi/issues/802


Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>