drm/doc/rfc: i915 new parallel submission uAPI plan

drm/doc/rfc: i915 new parallel submission uAPI plan

Add entry for i915 new parallel submission uAPI plan.

v2:
 (Daniel Vetter):
  - Expand logical order explaination
  - Add dummy header
  - Only allow N BBs in execbuf IOCTL
  - Configure parallel submission per slot not per gem context
v3:
 (Marcin Ślusarz):
  - Lot's of typos / bad english fixed
 (Tvrtko Ursulin):
  - Consistent pseudo code, clean up wording in descriptions
v4:
 (Daniel Vetter)
  - Drop flags
  - Add kernel doc
  - Reword a few things / fix typos
 (Tvrtko)
  - Reword a few things / fix typos
v5:
 (Checkpatch)
  - Fix typos
 (Docs)
  - Fix warning

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tony Ye <tony.ye@intel.com>
CC: Carl Zhang <carl.zhang@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Tony Ye <tony.ye@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210629193511.124099-3-matthew.brost@intel.com