drm/i915: Relocate intel_ddi_get_buf_trans_*() functions

We'll want to use the intel_ddi_get_buf_trans_*() functions a bit
earlier in the file, so move them up. While at it start using them
in the iboost setup to get rid of the platform checks there.

v2: Rebase due to BDW FDI buf trans fix

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016145705.11780-2-ville.syrjala@linux.intel.com


Reviewed-by: James Ausmus <james.ausmus@intel.com>