ARM SoCs usually have their DRM/KMS devices on the platform bus, so add
support for this bus in order to allow use of the DRI_PRIME environment
variable with those devices.

While at it, also support the host1x bus, which is effectively the same
but uses an additional layer in the bus hierarchy.

Note that it isn't enough to support the bus that has the rendering GPU
because the loader code will also try to construct an ID path tag for a
scanout-only device if it is the default that is being opened.

The ID path tag for a device can be obtained by running udevadm info on
the device node, as shown in this example on NVIDIA Tegra:

	$ udevadm info /dev/dri/card0 | grep ID_PATH_TAG
	E: ID_PATH_TAG=platform-50000000_host1x

The corresponding OF_FULLNAME property, from which the ID_PATH_TAG is
constructed, can be found in the sysfs "uevent" attribute for the card0
device's parent:

	$ grep OF_FULLNAME /sys/devices/platform/50000000.host1x/drm/uevent
	OF_FULLNAME=/host1x@50000000

Similarily, /dev/dri/card1 corresponds to the GPU:

	$ udevadm info /dev/dri/card1 | grep ID_PATH_TAG
	E: ID_PATH_TAG=platform-57000000_gpu

and:

	$ grep OF_FULLNAME /sys/devices/platform/57000000.gpu/uevent
	OF_FULLNAME=/gpu@57000000

Changes in v2:
- avoid confusing pre-increment in strdup()
- add examples of tags to commit message

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>