drm: Clarify the role of plane_state argument to drm_simple update().

Like the atomic update hook it's wrapping, the plane_state is the old
one, and the new one is in plane->state.  Both msxfb and tinydrm use
it correctly, but I mistook it for the new state in pl111 due to its
naming.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170320233615.5242-3-eric@anholt.net
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>