gpu: host1x: Don't fail on NULL bo physical address

Pinning a Host1x BO currently cannot fail and zero is a valid address
for a BO when IOMMU is enabled. To avoid false errors remove checks
for NULL BO physical addresses.

Fixes: 404bfb78 ("gpu: host1x: Add IOMMU support")
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>