st/mesa: do proper error checking for u_upload_alloc() calls

st/mesa: do proper error checking for u_upload_alloc() calls

We weren't properly checking the return value of these calls (and
calls to u_upload_data()) to detect OOM errors.

Note: This is a candidate for the 9.0 branch.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>