clutter/stage-cogl: Simplify redraw function

Spotted while adding tracing to swap buffers, we only enter
the first part of the if condition when use_clipped_redraw
is TRUE, so it's pretty safe to assume it's TRUE.

!197
4 jobs for !197 with gbsneto/tracing in 35 seconds
detached
Status Job ID Name Coverage
  Review
failed #304165
check-commit-log

00:00:35

 
  Build
skipped #304166
build-mutter
 
  Test
skipped #304168
can-build-gnome-shell
skipped #304167
test-mutter
 
Name Stage Failure
failed
check-commit-log Review
 * [new tag]         3.33.1     -> 3.33.1
Checking out 7183a002 as refs/merge-requests/197/head...

Skipping Git submodules setup
$ ./.gitlab-ci/check-commit-log.sh
From https://gitlab.gnome.org/GNOME/mutter
* branch master -> FETCH_HEAD
Missing merge request or issue URL on commit a11fe01b
ERROR: Job failed: exit code 1