st/mesa: make use of nir_link_constant_varyings()

st/mesa: make use of nir_link_constant_varyings()

Shader-db results radeonsi (VEGA):

Totals from affected shaders:
SGPRS: 161464 -> 161368 (-0.06 %)
VGPRS: 86904 -> 86292 (-0.70 %)
Spilled SGPRs: 296 -> 314 (6.08 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 3618596 -> 3573852 (-1.24 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 26189 -> 26276 (0.33 %)
Wait states: 0 -> 0 (0.00 %)

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