drm/i915: Pass the correct encoder to intel_ddi_clk_select() with MST

We're supposed to pass the primary DP encoder to intel_ddi_clk_select(),
not the fake MST encoder. Do so.

There's no real bug here though, since intel_ddi_clk_select() only
checks if the encoder type is EDP (which it isn't for either the
primary DP encoder or the fake MST encoder), and it gets the DDI port
via intel_ddi_get_encoder_port() (which knows how to do the
fake->primary->port dance itself).

Fixes: e404ba8d

 ("drm/i915: Setup DDI clk for MST on SKL")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449597590-6971-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>