drm/doc: add rfc section for small BAR uapi

drm/doc: add rfc section for small BAR uapi

Add an entry for the new uapi needed for small BAR on DG2+.

v2:
  - Some spelling fixes and other small tweaks. (Akeem & Thomas)
  - Rework error capture interactions, including no longer needing
    NEEDS_CPU_ACCESS for objects marked for capture. (Thomas)
  - Add probed_cpu_visible_size. (Lionel)
v3:
  - Drop the vma query for now.
  - Add unallocated_cpu_visible_size as part of the region query.
  - Improve the docs some more, including documenting the expected
    behaviour on older kernels, since this came up in some offline
    discussion.
v4:
  - Various improvements all over. (Tvrtko)

v5:
  - Include newer integrated platforms when applying the non-recoverable
    context and error capture restriction. (Thomas)

Mesa: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16739
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Cc: mesa-dev@lists.freedesktop.org
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Acked-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220629174350.384910-1-matthew.auld@intel.com