drm/ast: Program mode for AST DP in atomic_mode_set

The CRTC's atomic_flush function contains code to program the
display mode to the AST DP chip. Move the code to the encoder's
atomic_mode_set callback. The DRM atomic-modesetting code invoke
this callback as part of the atomic commit.

v2:
- fix typos in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-3-tzimmermann@suse.de