drm/ast: Move mode-setting code into mode_set_nofb CRTC helper

Do all mode setting in ast_crtc_helper_mode_set_nofb(), which
always runs after disabling the CRTC and before programming the
planes. Removes implicit synchronization between the CRTC's
atomic disable, enable and the vertical retrace.

Display-mode updates require HW cursors to be disabled. The HW
cursor only picks up changes at vertical retrace periods. So the
CRTC's atomic_disable helper waited for the retrace to delay any
following mode-setting operations, which then happened in
atomic_enable. See [1] for a description of the problem.

With the CRTC helper callback mode_set_nofb, we can now synchronize
and reprogram in the same place. As it always runs before the plane
update, the plane code can be reordered with the CRTC's later
atomic_enable et al.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/series/79914/

 # 1
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-4-tzimmermann@suse.de