oniro-openharmony-linux: allow running the system without hdmi connected

enable HDMI_FORCE_HOTPLUG to allow running the system without
an hdmi display connected.
HDMI_FORCE_HOTPLUG configuration does not work with vc4-kms-v3d dtoverlay,
therefore using vc4-fkms-v3d instead.

Signed-off-by: Francesco Pham <francesco.pham@huawei.com>