vc4: add hash table look-up for exported dmabufs

vc4: add hash table look-up for exported dmabufs

It is necessary to reuse existing BOs when dmabufs are imported. There
are 2 cases that need to be handled. dmabufs can be created/exported and
imported by the same process and can be imported multiple times.
Copying other drivers, add a hash table to track exported BOs so the
BOs get reused.

v2: Whitespace fixup (by anholt)

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 9ace2c13550609dfe78164f104500d438821f383)