drm/i915/uapi: implement object placement extension

drm/i915/uapi: implement object placement extension

Add new extension to support setting an immutable-priority-list of
potential placements, at creation time.

If we use the normal gem_create or gem_create_ext without the
extensions/placements then we still get the old behaviour with only
placing the object in system memory.

v2(Daniel & Jason):
    - Add a bunch of kernel-doc
    - Simplify design for placements extension

Testcase: igt/gem_create/create-ext-placement-sanity-check
Testcase: igt/gem_create/create-ext-placement-each
Testcase: igt/gem_create/create-ext-placement-all
Signed-off-by: Matthew Auld <[email protected]>
Signed-off-by: CQ Tang <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Lionel Landwerlin <[email protected]>
Cc: Jordan Justen <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Kenneth Graunke <[email protected]>
Cc: Jason Ekstrand <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Kenneth Graunke <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]