We currently support two modes of operation for RW PBO buffers: one
that allocates a pack buffer with GL memory and one that uses system
memory when the former is not supported.
Since allocation with system memory is less likely to fail, add a
fallback to system memory when GL memory failed instead of bailing
out.
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
(cherry picked from commit 8c4e8d9eff)