drm/amdgpu: Use drm_connector_for_each_possible_encoder()

Use drm_connector_for_each_possible_encoder() for iterating
connector->encoder_ids[]. A bit more convenient not having
to deal with the implementation details.

v2: Replace drm_for_each_connector_encoder_ids() with
    drm_connector_for_each_possible_encoder() (Daniel)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628131315.14156-5-ville.syrjala@linux.intel.com