drm/i915: ironlake_write_eld code cleanup

Use _PIPE macro to get correct register definition for IBX/CPT, discard
old variable "i" way.

Signed-off-by: Wang Xingchao <xingchao.wang@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Added the DIP_PORT_SEL #define from a preceeding patch in the
series that needs more work.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>