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.h

@@ -25,6 +25,8 @@
 #include "backends/meta-output.h"
 #include "backends/native/meta-monitor-manager-kms.h"
 
+void meta_output_kms_set_underscan (MetaOutput *output);
+
 void meta_output_kms_set_power_save_mode (MetaOutput *output,
                                           uint64_t    state);