xf86drm: Add a human readable representation for format modifiers

Introduces two new methods to retrieve a human readable representation of a
format modifier:

drmGetFormatModifierName() - returns a format modifier as a string,
from a token modifier
drmGetFormatModifierVendor() - returns the vendor as a string, from a
token modifier

and the fourcc_mod_get_vendor macro that returns the vendor.

New format modifiers added in drm_fourcc.h uapi kernel header should be
sync'ed up with libdrm and should include a human readable
representation for that format modifier, in order to display it
correctly as a string.

That happens with the help of a python script that reads up drm_fourcc
header file and outputs a static table comprised of token modifiers
alongside a vendor table (Suggested-by Simon Ser <contact@emersion.fr>).

The reason for doing it in libdrm is to have a unified place instead of each
user of libdrm having a way to keep track of the format modifiers.

With this patch, modetest has also been modified to make use of it.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>