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 4 minutes and 48 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #314357
check-commit-log

00:00:24

 
  Build
passed #314358
build-mutter

00:01:25

 
  Test
passed #314360
can-build-gnome-shell

00:01:31

passed #314359
test-mutter

00:02:58