drm/i915/userptr: Enable read-only support on gen8+

On gen8 and onwards, we can mark GPU accesses through the ppGTT as being
read-only, that is cause any GPU write onto that page to be discarded
(not triggering a fault). This is all that we need to finally support
the read-only flag for userptr!

v2: Check default address space for read only support as a proxy for the
user context/ppgtt.

Testcase: igt/gem_userptr_blits/readonly*
Signed-off-by: Chris Wilson <[email protected]>
Cc: Jon Bloomfield <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Reviewed-by: Jon Bloomfield <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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