gpu: host1x: Check whether size of unpin isn't 0

[ Upstream commit ec589232 ]

Only gather pins are mapped by the Host1x driver, regular BO relocations
are not. Check whether size of unpin isn't 0, otherwise IOVA allocation at
0x0 could be erroneously released.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>