egl_dri2: Only clear EGL_PIXMAP_BIT if DRI config is double buffered

We don't want to set the pixmap bit in the EGL config if the DRI
config we're adding is a double buffered config.  However, don't clear
any other bits the platform might pass in in the surface_type
argument.
diff --git a/src/egl/drivers/dri2/egl_dri2.c b/src/egl/drivers/dri2/egl_dri2.c
index 0099a0d..6c1c17d 100644
--- a/src/egl/drivers/dri2/egl_dri2.c
+++ b/src/egl/drivers/dri2/egl_dri2.c
@@ -259,8 +259,10 @@
       return NULL;
    }
 
-   conf->base.SurfaceType |= surface_type & (!double_buffer ? EGL_PIXMAP_BIT:
-         (EGL_WINDOW_BIT | EGL_PBUFFER_BIT | EGL_SWAP_BEHAVIOR_PRESERVED_BIT));
+   if (double_buffer)
+      surface_type &= ~EGL_PIXMAP_BIT;
+
+   conf->base.SurfaceType |= surface_type;
 
    return conf;
 }