drm/msm: Fix iommu_map_sgtable() return value check and avoid WARN

In the Linux kernel, the following vulnerability has been resolved: drm/msm: Fix iommu_map_sgtable() return value check and avoid WARN Commit "iommu: return full error code from iommu_map_sg[_atomic]()" changed iommu_map_sgtable() to return an ssize_t and negative values in error cases, rather than a size_t and a zero. Store the return value in the appropriate type and in case of error, return it rather than WARNing. Patchwork: https://patchwork.freedesktop.org/patch/719685/