radeonsi: Always pre-load separate VGPRs for centroid vs. center interpolation

radeonsi: Always pre-load separate VGPRs for centroid vs. center interpolation

The LLVM R600 backend currently always uses separate VGPRs for these.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68162
(Centroid interpolation is identical to center interpolation without
multisampling, so the shader hardware was only pre-loading one set of
interpolation coefficients, and the pixel shader code was using
uninitialized values as the centroid interpolation coefficients)

Cc: mesa-stable@lists.freedesktop.org
Tested-by: Laurent Carlier <lordheavym@gmail.com>