Add shared DRM platform for BSD

FreeBSD now has a port of the nvidia-drm.ko kernel module. In order to get PRIME render offload working with FreeBSD, we need a platform definition to implement the basic DRM device probing for the X server to be able to initialize the device.

This change simply moves hw/xfree86/os-support/linux/lnx_platform.c to the shared folder and plumbs it into both BSD and Linux builds.