winsys/drm: Add swrast.

The swrast winsys always returns NULL when drm_api_create is called.
diff --git a/docs/egl.html b/docs/egl.html
index 5561d1a..efc7b1e 100644
--- a/docs/egl.html
+++ b/docs/egl.html
@@ -82,6 +82,15 @@
 tracker.</p>
 
 </li>
+
+<li><code>--enable-gallium-swrast</code>
+
+<p>This option is not specific to EGL.  But if there is no driver for your
+hardware, or you are experiencing problems with the hardware driver, you can
+enable the swrast DRM driver.  It is a dummy driver and EGL will fallback to
+software rendering automatically.</p>
+
+</li>
 </ul>
 
 <h3>OpenGL</h3>
@@ -159,6 +168,7 @@
 <li><code>egl_&lt;dpy&gt;_i965</code></li>
 <li><code>egl_&lt;dpy&gt;_radeon</code></li>
 <li><code>egl_&lt;dpy&gt;_nouveau</code></li>
+<li><code>egl_&lt;dpy&gt;_swrast</code></li>
 <li><code>egl_&lt;dpy&gt;_vmwgfx</code></li>
 </ul>