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.