backends/native: Add some KMS debug logging

Using the g_debug() macro. Set G_DEBUG_MESSAGES to "mutter" to activate
log.

GNOME/mutter#548
GNOME/mutter!525
4 jobs for !525 with wip/kms-transactions in 5 minutes and 40 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #329584
check-commit-log

00:00:37

 
  Build
passed #329585
build-mutter

00:01:52

 
  Test
passed #329587
can-build-gnome-shell

00:01:42

passed #329586
test-mutter

00:03:11