compositor-drm: use weston_view_is_opaque()

Implement drm_view_is_opaque() using weston_view_is_opaque(). Also, use
weston_view_is_opaque() directly in drm_output_propose_state(), with the
clipped_view.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>