freedreno/ir3: add helper to determine point-coord inputs

This will simplify a bit the logic for setting up vinterp/vprepl in the
driver backend, and also avoid it being a flag-day when we switch the
texcoord pipe cap.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5595>
2 files changed