drm/i915/dp: move intel_dp_prepare_link_train() call

Call it from the higher level function, as it will be shared between two
code paths.

Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c0fffbf7213c09e42be9875d38d3001e0026b063.1643878928.git.jani.nikula@intel.com