With the high-level helpers requiring outputs there's not point
in silently ignoring issues anymore. Complain about that if it
ever happens.

This reverts

commit 212b7137


Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Sep 14 21:03:38 2018 +0100

    lib/kms: Skip no-op display updates

which created an in my opinion serious API issue by silently dropping
possible errors on the floor. Instead of silently second guess what
the test might have wanted to do in the absence of display outputs
it's much better to be explicit, and enforce that.

v2: Improve commit message.

v3: Switch to an assert and update comments, to make it clear this is
igt_display api abuse (Arek).

Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v2)
Acked-By: Antonio Argenziano <antonio.argenziano@intel.com>
Acked-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>