Retry get_fb_ptr in get_fb

If get_fb_ptr returns NULL, try again after pixmap_get_handle, it should
work then.

Fixes spurious Present page flipping failures using "normal" pixmaps
which aren't shared with direct rendering clients, e.g. with a
compositor using the RENDER extension.

Bugzilla: https://bugs.freedesktop.org/110417


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>