drm/i915: Name the "iboost bit"

Give a proper name for the SKL DDI_BUF_TRANS iboost bit.

Cc: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-3-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>