drm: add atomic_set_property wrappers

As we add properties for all the standard plane/crtc/connector
attributes (in preperation for the atomic ioctl), we are going to want
to handle core state in core (rather than per driver).  Intercepting the
core properties will be easier if the atomic_set_property vfuncs are not
called directly, but instead have a mandatory wrapper function (which
will later serve as the point to intercept core properties).

v2: more verbose comments and copypasta comment fix

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>