drm/i915/cnl; Add macro to get PORT_TX register

This patch creates a new macro to get PORT_TX register for any given DW.
This removes the need of defining register address for each port & DW.

Changes since V1:
 - Use underscope prefix, as macro isn't returning an mmio reg(Lucas)
 - Merge patch 1 & 2 of the series
Changes since V2:
 - remove _MMIO_PORT6_LN macro (Rodrigo)

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180314080653.9444-2-mahesh1.kumar@intel.com