drm/i915: Rename defines for selection of ddi buffer translation slot

drm/i915: Rename defines for selection of ddi buffer translation slot

Renaming the HSW-specific macros for ddi buffer translation slot to denote the
slot and not the vswing/pre-emph values as they are platform-dependent.

This patch is based on top of the patch series for renaming the DP training
vswing/pre-emph defines:
http://lists.freedesktop.org/archives/intel-gfx/2014-August/050407.html

v2: Creating single macro with argument for slot number (Damien)
v3: Adding macro for num of translation entries (Damien)

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>