modesetting: Explicitly #include "mi.h"

For the miClearDrawable prototype. Apparently it doesn't get pulled in
for some build configurations, breaking the build.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

hw/xfree86/drivers/modesetting/drmmode_display.c

@@ -37,6 +37,7 @@
 #include "inputstr.h"
 #include "xf86str.h"
 #include "X11/Xatom.h"
+#include "mi.h"
 #include "micmap.h"
 #include "xf86cmap.h"
 #include "xf86DDC.h"