drm/i915/selftests: Add basic sanitychecks for execlists

drm/i915/selftests: Add basic sanitychecks for execlists

Before adding a new feature to execlists submission, we should endeavour
to cover the baseline behaviour with selftests. So start the ball
rolling.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Michał Winiarski <[email protected]>
CC: Michel Thierry <[email protected]>
Cc: Jeff McGee <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]