drm/qxl: kill unused bo wait wrapper

This wasn't used anywheere

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923030454.362731-3-airlied@gmail.com