drm/ast: astdp: Replace ast_dp_set_on_off()

Replace ast_dp_set_on_off() with ast_dp_set_enable(). The helper's
new name reflects the performed operation. If enabling fails, the
new helper prints a warning. The code that waits for the programmed
effect to take place is now located in __ast_dp_wait_enable().

Also align the register constants with the rest of the code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911115347.899148-5-tzimmermann@suse.de