drm/doc: Make igts for cross-driver stuff strongly suggested

Compared to the RFC[1] no changes to the patch itself, but igt moved
forward a lot:

- gitlab CI builds with: reduced configs/libraries, arm cross build
  and a sysroot build (should address all the build/cross platform
  concerns raised in the RFC discussions).

- tests reorganized into subdirectories so that the i915-gem tests
  don't clog the main/shared tests directory anymore

- quite a few more non-intel people contributing/reviewing/committing
  igt tests patches.

I think this addresses all the concerns raised in the RFC discussions,
and assuming there's enough Acks and no new issues that pop up, we can
go ahead with this.

v2:
- Use "should" (in the usual RFC sense) to make it clear that in the
  end this is all up to reviewer's discretion, as usual (Jani).
- Also in the title s/mandatory/strongly suggested/ (me)
- Make it clear we're not going to block features if a testcase is not
  feasible, given hw and state of igt, both having some good gaps in
  what can be tested (Harry, Eric, Sean, ...).

1: https://patchwork.kernel.org/patch/10648851/


Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Eric Anholt <eric@anholt.net>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: "Wentland, Harry" <Harry.Wentland@amd.com>
Cc: Brian Starkey <Brian.Starkey@arm.com>
Reviewed-by: Eric Anholt <eric@anholt.net> (v1)
Acked-by: Petri Latvala <petri.latvala@intel.com>
Acked-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Sean Paul <sean@poorly.run>
Acked-by: "Wentland, Harry" <Harry.Wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Brian Starkey <brian.starkey@arm.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190128172258.9585-1-daniel.vetter@ffwll.ch