st/mesa: Replace UsesStreams by ActiveStreamMask for GS

Some drivers need to know which streams are used by a geometry
shader. Adding a mask of active streams makes the use of
UsesStreams superfluous as it's the equivalent of:

    ActiveStreamMask != (1 << 0)

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5984>

src/compiler/shader_info.h

@@ -234,8 +234,8 @@ typedef struct shader_info {
          /** Whether or not this shader uses EndPrimitive */
          bool uses_end_primitive:1;
 
-         /** Whether or not this shader uses non-zero streams */
-         bool uses_streams:1;
+         /** The streams used in this shaders (max. 4) */
+         uint8_t active_stream_mask:4;
       } gs;
 
       struct {