drm/i915: Enable DP port earlier

Bspec says we should enable the DP port before enabling panel power,
and that the port must be enabled with training pattern 1. Do so.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>