Add functions to check core plane/crtc state.

v2: comments, int-overflow checks, call from core rather than
    helpers to be sure drivers can't find a way to bypass core
    checks
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>