drm/atomic-helper: Unexport drm_atomic_helper_best_encoder

drm/atomic-helper: Unexport drm_atomic_helper_best_encoder

It's the default. The exported version was kinda a transition state,
before we made this the default.

To stop new atomic drivers from using it (instead of just relying on
the default) let's unexport it.

v2: rename the default implementation to a more fitting name and add a
comment (Laurent)

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Pierre-Hugues Husson <phh@phh.me>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-3-daniel.vetter@ffwll.ch