iris: Enable PIPE_CAP_LEGACY_MATH_RULES.

Now that TTN hooks this up to use_legacy_math_rules, we can flip the
switch and gallium nine can get the desired behavior from the hardware
instead of emitting math workarounds.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16176>

src/gallium/drivers/iris/iris_screen.c

@@ -292,6 +292,7 @@ iris_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_MIXED_COLOR_DEPTH_BITS:
    case PIPE_CAP_FENCE_SIGNAL:
    case PIPE_CAP_IMAGE_STORE_FORMATTED:
+   case PIPE_CAP_LEGACY_MATH_RULES:
       return true;
    case PIPE_CAP_PREFER_BACK_BUFFER_REUSE:
       return false;