modesetting: Call glamor_finish from drmmode_crtc_set_mode

This makes sure any pending drawing to a new scanout buffer will be
visible from the start.

This makes the finish call in drmmode_copy_fb superfluous, so remove it.

Reviewed-by: Adam Jackson <ajax@redhat.com>
(Cherry picked from commit c66c548e)
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>