svga: check for NaN shader immediates

The svga device doesn't handle them.  Replace with zeros.
Fixes several piglit tests, such as "glsl-const-builtin-inversesqrt".

Reviewed-by: Reviewed-by: José Fonseca <jfonseca@vmware.com>
(cherry picked from commit 3cb491534493a52e9a88cb88d31727569afb8167)