drm/i915: Split intel_ddi_pre_enable() into DP and HDMI versions

Split intel_ddi_pre_enable() into encoder type specific versions that
don't depend on crtc_state. The necessary parameters are passed as
function arguments. This split will be necessary for implementing DP
upfront link training.

v3:
* Rebased onto latest kernel (Manasi)
v2:
* Rebased onto kernel v4.7 (Jim)

Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>