libdrm

Our libdrm, in base and pkgsrc, currently only handles PCI graphics
controllers.

I have got some local changes to it to get it to default to identifying
a device as a 'platform' one if it isn't PCI. This will be used for the
on chip GPU on ARM systems, should also eventually work for MIPS and
RISC-V.

The default code in drmGetDeviceFromDevId() calls readdir(3) on the
"/dev/dri" directory to get a list of the devices then prunes this
down to one on each bus.

We want the retained device node to be the one for the framebuffer, Mesa
expects this.

It is easy enough to make the GPU device driver have a later fdt pass
number than the framebuffer driver in the kernel config to fix which
one will be card0 and which card1.

The unsorted order of the nodes in the directory will depend on when
they were created though, this can mean you end up with the retained
device node being for the GPU itself and Mesa initialization fails.

I have made this work on my test evbarm systems by deleting all the
nodes in /dev/dri then running:

# sh MAKEDEV drm1
# sh MAKEDEV drm0

This means that /dev/dri/card0 gets retained in the pruned list of
devices as the one to use and everything works.

We need something more robust though, any ideas?