gallium: expose ARB_map_buffer_alignment on Radeon

Reviewed-by: Brian Paul <brianp@vmware.com>

v2: update relnotes-9.1
v3: use align_malloc and align_free for malloced buffers in r300g
v4: document the new CAP in the docs

src/mesa/state_tracker/st_extensions.c

@@ -637,4 +637,10 @@ void st_init_extensions(struct st_context *st)
    }
    if (st->options.force_glsl_extensions_warn)
 	   ctx->Const.ForceGLSLExtensionsWarn = 1;
+
+   ctx->Const.MinMapBufferAlignment =
+      screen->get_param(screen, PIPE_CAP_MIN_MAP_BUFFER_ALIGNMENT);
+   if (ctx->Const.MinMapBufferAlignment >= 64) {
+      ctx->Extensions.ARB_map_buffer_alignment = GL_TRUE;
+   }
 }