i965: new VS: Use output_reg[] to find NDC and HPOS registers.

i965: new VS: Use output_reg[] to find NDC and HPOS registers.

Previously, emit_vue_header_gen4() used local variables to keep track
of which registers were storing the NDC and HPOS.  This patch uses the
output_reg[] array instead, so that the code that manipulates NDC and
HPOS can be more easily refactored.

Reviewed-by: Eric Anholt <eric@anholt.net>