kms-plane: Restore adding format fallbacks

There were fallbacks in place in case IN_FORMATS didn't yield any usable
formats: the formats in the drmModePlane struct, and a hard coded array.
The lack of these fallbacks in place could result in a segfault as code
using the supported plane formats assumed there were at least something
in there.

GNOME/mutter!662
5 jobs for !662 with wip/restore-drm-format-fallbacks in 9 minutes and 19 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #339761
check-commit-log

00:00:38

 
  Build
passed #339762
build-mutter

00:02:08

passed #339763
build-without-native-backend

00:02:07

 
  Test
passed #339765
can-build-gnome-shell

00:04:32

passed #339764
test-mutter

00:06:32