drm/ast: Read encoder possible-CRTC mask from drm_crtc_mask()

Read the encoder's possible-CRTC mask from the involved CRTC instead
of hard-coding it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-8-tzimmermann@suse.de