freedreno/ir3: handle flat bypass for a4xx

freedreno/ir3: handle flat bypass for a4xx

We may not need this for later a4xx patchlevels, but we do at least need
this for patchlevel 0.  Bypass bary.f for fetching varyings when flat
shading is needed (rather than configure via cmdstream).  This requires
a special dummy bary.f w/ (ei) flag to signal to scheduler when all
varyings are consumed.  And requires shader variants based on rasterizer
flatshade state to handle TGSI_INTERPOLATE_COLOR.

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