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>
12 jobs for bug-110417 in 1 minute and 14 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #259494

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  Build
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passed xserver-1.14 #259498

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passed xserver-1.18 #259502

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passed xserver-1.18-clang #259503

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passed xserver-1.20 #259495

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passed xserver-1.20-clang #259496

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