modesetting: Use glamor_clear_pixmap in drmmode_clear_pixmap

Should be slightly more efficient.

Reviewed-by: Adam Jackson <ajax@redhat.com>
3 jobs for modesetting-intermittent-garbage in 5 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #1012267
debian-testing

00:00:25

 
  Build And Test
passed #1012268
autotools-build-and-test

00:05:29

passed #1012269
meson-build-and-test

00:01:34