drm/i915: Add MMIO helper for 6 ports with different offsets.

drm/i915: Add MMIO helper for 6 ports with different offsets.

Also new registers can have different mmio offsets
per different lane per port.

v2: Use _PICK as PORT3 instead of creating a new
    macro with if per port.
v3: Use _PICK directly on MMIO_PORT6. While MMIO_PORT
    isn't flexible enough let's continue with MMIO_PORT6
    as we have MMIO_PORT3.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497047175-27250-8-git-send-email-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>