gallium/dri: Use per-screen DRI extension list

Some DRI extension features are enabled/disabled based on capabilities of the gallium pipe_screen associated with the DRI screen. Additionally, the list of extensions enabled also varied based on features requested by the screen creator. However, prior to this change the extension list and extension definition structures within it were global variables, meaning the last screen initialized ended up defining the DRI capabilities of all screens.

This change instead stores a copy of the extensions which vary per screen, as well as a copy of the extension list itself in the gallium DRI screen structure, allowing them to vary per screen.

Closes drm/nouveau#9 (closed)