r600g: avoid useless shader rebuild at draw call

r600g: avoid useless shader rebuild at draw call

Avoid rebuilding constant shader state at each draw call,
factor out spi update that might change at each draw call.
Best would be to update spi only when revealent states
change (likely only flat shading & sprite point).

Signed-off-by: Jerome Glisse <jglisse@redhat.com>