panfrost: Increase the limit for blend shader variants

Qt uses blend constants to set text colour, this will allow more
colours onscreen before thrashing happens.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16343>

src/panfrost/lib/pan_blend.h

@@ -90,7 +90,7 @@ struct pan_blend_shader_variant {
         unsigned work_reg_count;
 };
 
-#define PAN_BLEND_SHADER_MAX_VARIANTS 16
+#define PAN_BLEND_SHADER_MAX_VARIANTS 32
 
 struct pan_blend_shader {
         struct pan_blend_shader_key key;