drm: Don't force all planes to be added to the state due to zpos

We don't want all planes to be added to the state whenever a
plane with fixed zpos gets enabled/disabled. This is true
especially for eg. cursor planes on i915, as we want cursor
updates to go through w/o throttling. Same holds for drivers
that don't support zpos at all (i915 actually falls into this
category right now since we've not yet added zpos support).

Allow drivers more freedom by letting them deal with zpos
themselves instead of doing it in drm_atomic_helper_check_planes()
unconditionally. Let's just inline the required calls into all
the driver that currently depend on this.

v2: Inline the stuff into the drivers instead of adding another
    helper, document things better (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Lyude <cpaul@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: stable@vger.kernel.org
Fixes: 44d1240d

 ("drm: add generic zpos property")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476111056-12734-1-git-send-email-ville.syrjala@linux.intel.com