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 <[e-mail]>
Cc: Arkadiusz Hiler <[e-mail]>
Cc: Liviu Dudau <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: Eric Anholt <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Daniel Stone <[e-mail]>
Cc: "Wentland, Harry" <[e-mail]>
Cc: Brian Starkey <[e-mail]>
Reviewed-by: Eric Anholt <[e-mail]> (v1)
Acked-by: Petri Latvala <[e-mail]>
Acked-by: Arkadiusz Hiler <[e-mail]>
Acked-by: Sean Paul <[e-mail]>
Acked-by: "Wentland, Harry" <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Acked-by: Liviu Dudau <[e-mail]>
Acked-by: Jani Nikula <[e-mail]>
Acked-by: Brian Starkey <[e-mail]>
Acked-by: Tomi Valkeinen <[e-mail]>
Acked-by: Boris Brezillon <[e-mail]>
Acked-by: Alex Deucher <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/drm-uapi.rst

···
 Testing and validation
 ======================
 
+Testing Requirements for userspace API
+--------------------------------------
+
+New cross-driver userspace interface extensions, like new IOCTL, new KMS
+properties, new files in sysfs or anything else that constitutes an API change
+should have driver-agnostic testcases in IGT for that feature, if such a test
+can be reasonably made using IGT for the target hardware.
+
 Validating changes with IGT
 ---------------------------