[Nouveau] Winsys changes in mesa/gallium-0.2

[Younes Manton]

I made some changes to the Nouveau drm winsys so it could be shared
between Mesa and VL. Mostly this involved moving all the common stuff
into common/ and adding dri/.

Compiles and runs the stuff in progs/ like before as far as I know,
but if anyone actually uses the Mesa driver for anything non-trivial
I'd appreciate knowing if this broke anything for you. :)

* The code in common/ compiles to libnouveaudrm.a and the code in dri/
compiles to nouveau_dri.so and links with common.

* Structs nouveau_screen and nouveau_context are now in common and
only contain generic DRM stuff.

* Structs nouveau_screen_dri and nouveau_screen_dri in dri/ contain
the DRI/Mesa specific bits and use nouveau_screen and nouveau_context
as base.

* The dri/ dir also contains nouveau_swapbuffers.c which has all the
swapping functions plus nouveau_contended_lock and
nouveau_flush_frontbuffer. These are DRI/Mesa specific so I couldn't
keep them generic.

* The vl winsys contains the same 3 files, nouveau_screen_vl,
nouveau_context_vl, and nouveau_swapbuffers which basically do the
same as the dri/ dir if anyone is interested.