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>
(cherry picked from commit a24a786f)
3 jobs for server-1.20-build-fixes in 5 minutes and 2 seconds
Status Job ID Name Coverage
  Docker Image
passed #1358277
debian-testing

00:00:30

 
  Build And Test
passed #1358278
autotools-build-and-test

00:04:32

passed #1358279
meson-build-and-test

00:01:18