intel: tell libdrm whether we want a cpu-ready or gpu-ready BO for regions.

intel: tell libdrm whether we want a cpu-ready or gpu-ready BO for regions.

This lets us avoid allocing new buffers for renderbuffers, finalized miptrees,
and PBO-uploaded textures when there's an unreferenced but still active one
cached, while also avoiding CPU waits for batchbuffers and CPU-uploaded
textures.  The size of BOs allocated for a desktop running current GL
cairogears on i915 is cut in half with this.

Note that this means we require libdrm 2.4.5.