iris: drop cache coherent cpu mapping for external BO

iris: drop cache coherent cpu mapping for external BO

We have to assume any external buffer could be used by the display HW.
In the case that buffer is also CPU mapped, we want to assume no cache
coherency as it is only available between GT & CPU, not display.

Many thanks to Michel Dänzer for the hint!

v2: Move cache coherent drop to bufmgr (Chris)

v3: Also make BO external if created with PIPE_BIND_SHARED (Eric)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2552
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4533>