xf86drm: Add a vendor function to decode the format modifier

As format modifiers can be encoded in quite complex forms, the static
table previously added is not sufficient to retrieve, extract and decode
the token formats to a human-readable string.  This patch introduces a
vendor specific callback which could be used to perform an additional
search to match up with vendor encoding scheme, which, will be used
first, before resorting to searching the static table.

With it, add support for decoding the ARM format modifiers.

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