i965: Make sure we don't use CPU maps for the scanout buffer.

i965: Make sure we don't use CPU maps for the scanout buffer.

Using an incoherent CPU map on the active scanout buffer is really
sketchy - we may need extra flushing via GEM_SW_FINISH, or using
drmModeDirtyFB() and kernel commit a6a7cc4b7db6d (4.10+).

Chris suggests "never ever do that", which seems like a wise plan!

intel_miptree_map_raw() uses CPU maps on linear buffers.

Having a linear scanout buffer should be really rare, and mapping the
front buffer should be similarly rare.  Together, it should basically
never happen.  But, in case it does somehow...make sure that mapping
the scanout buffer always goes through an uncached GTT map.

v2: Add a giant comment written by Chris Wilson.

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