compositor-drm: allow mode frequency selection

As an option, allow to specify a mode (from the configuration file) by
its refresh rate.
Example of valid syntax:
- "mode=1920x1080"    Select a 1920x1080 mode, refresh rate undefined.
- "mode=1920x1080@60" Select the (or one of the) 1920x1080 60 Hz mode.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/weston.ini.in b/weston.ini.in
index d837fb5..71e879a 100644
--- a/weston.ini.in
+++ b/weston.ini.in
@@ -55,7 +55,7 @@
 
 #[output]
 #name=X1
-#mode=1024x768
+#mode=1024x768@60
 #transform=flipped-90
 
 #[libinput]