r600g: move selecting the pixel shader earlier.

r600g: move selecting the pixel shader earlier.

In order to detect that a pixel shader has a prim id
input when we have no geometry shader we need to reorder
the shader selection so the pixel shader is selected
first, then the vertex shader key can take into account
the primitive id input requirement and lack of geom shader.

Reviewed-by: Glenn Kennard <glenn.kennard@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>