nvc0: fix valid range for shader buffers

nvc0: fix valid range for shader buffers

When offset != 0, the valid range was wrong because the second
argument of util_range_add() is end, not size.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>