freedreno/a3xx: add shader variants

freedreno/a3xx: add shader variants

Start putting in place infrastructure to deal with multiple shader
variants.  Initially we'll use this for two sided color (frag) and
binning pass (vert) shaders.  Possibly need for others later (such
as YUV vs RGB eglImage?).

Signed-off-by: Rob Clark <robclark@freedesktop.org>