iris: fix PIPE_RESOURCE_PARAM_STRIDE for buffers
Iris calls iris_resource_get_param with PIPE_RESOURCE_PARAM_STRIDE internally now when exporting memory objects. OpenCL's gl_sharing allows to export buffers as well, which do not have strides.
This fixes the assert being hit there for buffers.
Fixes: 83170315 ("iris: Use resource_get_param in resource_get_handle")