Use custom page flip function when retrying failed flips

When using its EGLStream-based presentation path with the proprietary NVIDIA driver, mutter will use a different function to process page flips - custom_egl_stream_page_flip. If that fails due to an EBUSY error, it will attempt to retry the flip. However, when retrying, it unconditionally uses the libdrm-based path. In practice, this causes a segfault when attempting to access plane_assignments->fb_id, since plane_assignments will be NULL in the EGLStream case. The issue can be reproduced reliably by VT-switching away from GNOME and back again while an EGL application is running.

This patch has mutter also use the custom page flip function when retrying the failed flip.