kwin: Don't hold an X server grab while painting the screen

According to Pierre-Loup Griffais (Plagman on IRC) this is causing
choppy VDPAU video playback when compositing is enabled.

Removing these calls doesn't seem to cause any regressions, and the
commit that added them doesn't give a reason. So let's just get rid
of them for now.

This commit should be cherry-picked to the stable branch if no one
else notices any regressions.