drm/vkms: Update vkms_composer_worker documentation

Since commit ba420afa ("drm/vkms: Bugfix racing hrtimer vblank
handle") the work is scheduled at vkms_vblank_simulate() and since
commit 5ef8100a ("drm/vkms: flush crc workers earlier in commit
flow") the work is flushed at vkms_atomic_commit_tail(). Update function
commment to reflect that.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220521191342.23520-1-andrealmeid@igalia.com