panfrost: Assign primitive_size.pointer only if writes_point_size() returns true

Checking vs->writes_point_size is not enough, as we might have a vertex
shader writing point size, but a primitive that's not MALI_POINT. That
currently works because emit_varying_descriptor() is called before the
primitive_size.constant field is update, but let's make the logic more
robust, just in case things are re-ordered at some point.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4083>

src/gallium/drivers/panfrost/pan_context.c

@@ -353,7 +353,7 @@ panfrost_default_shader_backend(struct panfrost_context *ctx)
         memcpy(&ctx->fragment_shader_core, &shader, sizeof(shader));
 }
 
-static bool
+bool
 panfrost_writes_point_size(struct panfrost_context *ctx)
 {
         assert(ctx->shader[PIPE_SHADER_VERTEX]);

src/gallium/drivers/panfrost/pan_context.h

@@ -278,6 +278,9 @@ panfrost_create_context(struct pipe_screen *screen, void *priv, unsigned flags);
 void
 panfrost_invalidate_frame(struct panfrost_context *ctx);
 
+bool
+panfrost_writes_point_size(struct panfrost_context *ctx);
+
 void
 panfrost_emit_for_draw(struct panfrost_context *ctx, bool with_vertex_data);
 

src/gallium/drivers/panfrost/pan_varyings.c

@@ -299,7 +299,7 @@ panfrost_emit_varying_descriptor(
                                        sizeof(float) * 4, vertex_count);
 
 
-        if (vs->writes_point_size)
+        if (panfrost_writes_point_size(ctx))
                 ctx->payloads[PIPE_SHADER_FRAGMENT].primitive_size.pointer =
                         panfrost_emit_varyings(ctx, &varyings[gl_PointSize],
                                                2, vertex_count);