mesa: replace gl_context->Multisample._Enabled with _mesa_is_multisample_enabled.

This removes any dependency on driver validation of the number of
framebuffer samples.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Brian Paul <brianp@vmware.com>

src/mesa/program/prog_statevars.c

@@ -502,7 +502,7 @@ _mesa_fetch_state(struct gl_context *ctx, const gl_state_index state[],
                minImplSize = ctx->Const.MinPointSizeAA;
                maxImplSize = ctx->Const.MaxPointSize;
             }
-            else if (ctx->Point.SmoothFlag || ctx->Multisample._Enabled) {
+            else if (ctx->Point.SmoothFlag || _mesa_is_multisample_enabled(ctx)) {
                minImplSize = ctx->Const.MinPointSizeAA;
                maxImplSize = ctx->Const.MaxPointSizeAA;
             }