drm/nouveau: Do not implement mode_set_base_atomic callback

drm/nouveau: Do not implement mode_set_base_atomic callback

Remove the implementation of the CRTC helper mode_set_base_atomic
from nouveau. It pretends to provide mode setting for kdb debugging,
but has been broken for some time.

Kdb output has been supported only for non-atomic mode setting since
commit 9c79e0b1d096 ("drm/fb-helper: Give up on kgdb for atomic drivers")
from 2017.

While nouveau provides non-atomic mode setting for some devices, kdb
assumes that the GEM buffer object is at a fixed location in video
memory. This has not been the case since
commit 4a16dd9d18a0 ("drm/nouveau/kms: switch to drm fbdev helpers")
from 2022. Fbdev-ttm helpers use a shadow buffer with a movable GEM
buffer object. Triggering kdb does therefore not update the display.

Hence remove the whole kdb support from nouveau.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Acked-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Link: https://patch.msgid.link/20251125130634.1080966-3-tzimmermann@suse.de