Use drm_connector_has_possible_encoder() for checking
whether the encoder has an associated connector.

v2: Replace the drm_for_each_connector_encoder_ids() loop
    with a simple drm_connector_has_possible_encoder() call

Cc: Rob Clark <robdclark@gmail.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628131315.14156-9-ville.syrjala@linux.intel.comReviewed-by: Alex Deucher <alexander.deucher@amd.com>