For atomic driver this is the default, no need to reimplement it. We
still need to keep the copypasta for not-atomic drivers though, since
no one polished the legacy crtc helpers as much as the atomic ones.

v2: amdgpu uses ->best_encoder internally, give it a local copy. It
might be a good idea to merge the connector and encoder into one
amdgpu_dm_sink structure, that might match DC internals better. At
least for non-DPMST outputs. Kudos to Ville for spotting this.

v3: Rebase onto a487411a ("drm/amd/display: Use DRM helper for
best_encoder").

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Tony Cheng <Tony.Cheng@amd.com>
Cc: "Leo (Sunpeng) Li" <sunpeng.li@amd.com>
Cc: Shirish S <shirish.s@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-2-daniel.vetter@ffwll.ch