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

drivers/gpu/drm/ast/ast_mode.c

@@ -1341,7 +1341,7 @@ static int ast_vga_output_init(struct ast_private *ast)
 	ret = drm_simple_encoder_init(dev, encoder, DRM_MODE_ENCODER_DAC);
 	if (ret)
 		return ret;
-	encoder->possible_crtcs = 1;
+	encoder->possible_crtcs = drm_crtc_mask(crtc);
 
 	ret = ast_vga_connector_init(dev, ast_vga_connector);
 	if (ret)