drm/i915: Split DP/eDP/FDI and HDMI/DVI DDI buffer programming apart

DDI buffer prorgramming works quite differently depending on
the mode of the DDI port (DP/eDP/FDI vs. HDMI/DVI). Let's split
the function that does the programming into two matching variants
as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-7-git-send-email-ville.syrjala@linux.intel.com