nv30: don't assert when running out of registers

nv30: don't assert when running out of registers

This happens with dEQP tests. The code doesn't at all protect against
this condition, so while unhandled, this is an expected situation.

Also avoid using more than the first 16 registers for nv3x vertex
programs.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>