drm/connector: Introduce a TV margins structure

The TV margins has been defined as a structure inside the
drm_connector_state structure so far. However, we will need it in other
structures as well, so let's move that structure definition so that it can
be reused.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/38b773b03f15ec7a135cdf8f7db669e5ada20cf2.1560783090.git-series.maxime.ripard@bootlin.com