drm/doc/rfc: i915 GuC submission / DRM scheduler

Add entry for i915 GuC submission / DRM scheduler integration plan.
Follow up patch with details of new parallel submission uAPI to come.

v2:
 (Daniel Vetter)
  - Expand explaination of why bonding isn't supported for GuC
    submission
  - CC some of the DRM scheduler maintainers
  - Add priority inheritance / boosting use case
  - Add reasoning for removing in order assumptions
 (Daniel Stone)
  - Add links to priority spec
v4:
 (Tvrtko)
  - Add TODOs section
 (Daniel Vetter)
  - Pull in 1 line from following patch
v5:
 (Checkpatch)
  - Fix typos

Cc: Christian König <christian.koenig@amd.com>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210629193511.124099-2-matthew.brost@intel.com