drm/doc/rfc: i915 DG1 uAPI

drm/doc/rfc: i915 DG1 uAPI

Add an entry for the new uAPI needed for DG1. Also add the overall
upstream plan, including some notes for the TTM conversion.

v2(Daniel):
  - include the overall upstreaming plan
  - add a note for mmap, there are differences here for TTM vs i915
  - bunch of other suggestions from Daniel
v3:
 (Daniel)
  - add a note for set/get caching stuff
  - add some more docs for existing query and extensions stuff
  - add an actual code example for regions query
  - bunch of other stuff
 (Jason)
  - uAPI change(!):
	- try a simpler design with the placements extension
	- rather than have a generic setparam which can cover multiple
	  use cases, have each extension be responsible for one thing
	  only
v4:
 (Daniel)
  - add some more notes for ttm conversion
  - bunch of other stuff
 (Jason)
  - uAPI change(!):
	- drop all the extra rsvd members for the region_query and
	  region_info, just keep the bare minimum needed for padding
v5:
(Jason)
  - for the upstream plan, add a requirement that we send the uAPI bits
    again for final sign off before turning it on for real
  - document how we intend to extend the rsvd bits for the region query
(Kenneth)
  - improve the comment for the smem+lmem mmap mode and caching

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Jon Bloomfield <jon.bloomfield@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210429103056.407067-1-matthew.auld@intel.com