mesa: replace for loop with bitshifting in supported_buffer_bitmask()

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/mesa/main/buffers.c

@@ -58,10 +58,7 @@ supported_buffer_bitmask(const struct gl_context *ctx,
 
    if (_mesa_is_user_fbo(fb)) {
       /* A user-created renderbuffer */
-      GLuint i;
-      for (i = 0; i < ctx->Const.MaxColorAttachments; i++) {
-         mask |= (BUFFER_BIT_COLOR0 << i);
-      }
+      mask = ((1 << ctx->Const.MaxColorAttachments) - 1) << BUFFER_COLOR0;
    }
    else {
       /* A window system framebuffer */