Add new dri-common abstraction to contain basic DRI-specific rules.
This refactoring is based on a decision to have set of dri-* abstractions for
fine grained control on case-by-case basis. While dri-common is included in X
abstraction by default, additional DRI-related abstractions can be introduced
(such as for enumerating graphics devices) while keeping them logically together
with same dri- prefix.
