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>
3 jobs for modesetting-mi.h in 6 minutes and 57 seconds
Status Job ID Name Coverage
  Docker Image
passed #1358271
debian-testing

00:00:26

 
  Build And Test
passed #1358272
autotools-build-and-test

00:06:31

passed #1358273
meson-build-and-test

00:01:57