backends/native: Move CRTC code to its own file

Move code dealing with MetaCrtcKms and related functionality to its
own file. Eventually, MetaCrtcKms should become a GObject based on
MetaCrtc, and this commit is in preparation for that.

https://bugzilla.gnome.org/show_bug.cgi?id=785381

src/backends/native/meta-output-kms.c

@@ -28,6 +28,7 @@
 #include <string.h>
 
 #include "backends/meta-crtc.h"
+#include "backends/native/meta-crtc-kms.h"
 #include "backends/native/meta-default-modes.h"
 #include "backends/native/meta-monitor-manager-kms.h"
 
@@ -61,6 +62,16 @@ typedef struct _MetaOutputKms
   gboolean has_scaling;
 } MetaOutputKms;
 
+void
+meta_output_kms_set_underscan (MetaOutput *output)
+{
+  if (!output->crtc)
+    return;
+
+  meta_crtc_kms_set_underscan (output->crtc,
+                               output->is_underscanning);
+}
+
 void
 meta_output_kms_set_power_save_mode (MetaOutput *output,
                                      uint64_t    state)