i965: Make a screen::aperture_threshold field.

i965: Make a screen::aperture_threshold field.

This is the threshold after which drm_intel_bufmgr_check_aperture_space
returns -ENOSPC, signalling that it thinks an execbuf is likely to fail
and we need to roll back and flush the batch.

We'll need this when we rewrite aperture space checking, shortly.
In the meantime, we can also use it in GLX_MESA_query_renderer.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>