glamor: Don't insert fbos from external objects into fbo cache

Mark fbos created from external buffers so that when the associated
pixmap is destroyed, they aren't put into the fbo cache for later
re-use and are instead freed immediately.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>