qxl: hook monitors_config updates into crtc, not encoder.

The encoder callbacks are only called in case the video mode changes.
So any layout changes without mode changes will go unnoticed.

Add qxl_crtc_update_monitors_config(), based on the old
qxl_write_monitors_config_for_encoder() function.  Hook it into the
enable, disable and flush atomic crtc callbacks.  Remove monitors_config
updates from all other places.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1544322
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180420071904.24276-4-kraxel@redhat.com