xf86drm: Add platform and host1x bus support

ARM SoCs usually have their DRM/KMS devices on the platform bus, so add
support for that to enable these devices to be used with the drmDevice
infrastructure.

NVIDIA Tegra SoCs have an additional level in the hierarchy and DRM/KMS
devices can also be on the host1x bus. This is mostly equivalent to the
platform bus.

v4:
- continue on error to process platform or host1x device

v3:
- guard Linux-specific sysfs parsing code with #ifdef __linux__

v2:
- be careful not to overflow the full name
- read compatible strings into device info

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>