drm/i915: Eliminate duplicated skl_get_buf_trans_dp()

skl_get_buf_trans_edp() effectively contains another copy of
skl_get_buf_trans_dp(). Remove the duplication and just call
skl_get_buf_trans_dp() from  skl_get_buf_trans_edp().

v2: Resolve conflicts due to KBL

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)