drm/i915: Fix SKL i_boost level

The i_boost level in the DDI translation tables are stored per level.
However, skl_ddi_set_iboos() would choose an entry of that table based
on the port argument.

Cc: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447247754-802-1-git-send-email-ander.conselvan.de.oliveira@intel.com