i965/bufmgr: Use write-combine mappings where available

Write-combine mappings give much better performance on writes than
uncached access through the GTT.

Improves performance of GFXBench 4's gl_driver2 benchmark at 1024x768
on Apollolake by 3.6086% +/- 0.674193% (n=15).

v2: (by Ken) Rebase on lockless mappings, map_count deletion, valgrind
    updates, potential for CPU/WC maps failing, and other changes.

v3: (by Ken and Chris Wilson)

    (Ken): Rebase on set_domain -> gem_wait
    (Chris): Fix up a failed CPU/WC mmaping with a GTT mapping

    Not all objects will be mappable for direct access by the CPU
    (either using WC/CPU or WC paths), for example, a dmabuf wrapping an
    object on a foreign device or an object wrapping access to stolen
    memory. Since either the physical pages are not known or even do not
    exist, we need to use the mediated, indirect access via the GTT. (If
    one day, the kernel does suddenly start providing mediated access
    via a regular WB/WC mmapping, we no longer need the fallback.)

v4: Avoid falling back for MAP_RAW (Chris).

Reviewed-by: Kenneth Graunke <[email protected]>

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