drm/i915: Centralize the SKL DDI A/E vs. B/C/D buf trans handling

drm/i915: Centralize the SKL DDI A/E vs. B/C/D buf trans handling

SKL DDI B/C/D only have 9 usable buf trans registers for DP/eDP. That
matches the normal DP buf trans tables, but the low vswing eDP tables
have 10 entries. Thus the eDP tables can only be used safely with DDI A
and E.

We try to catch cases where DDI B/C/D gets used with the wrong number of
entires in some parts of the code, but not everywhere. Let's move the
code to deal with that deeper into intel_ddi_get_buf_trans_edp(). And
for sake of symmetry do the same in intel_ddi_get_buf_trans_dp(). That
would also avoid explosions in the rather unlikely case that the DP
tables would get revised to 10 entries as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016145705.11780-9-ville.syrjala@linux.intel.com
Reviewed-by: James Ausmus <james.ausmus@intel.com>