drm/i915: Renaming DP training vswing pre emph defines

Rename the defines to have levels instead of values for vswing and
pre-emph levels as the values may differ in other scenarios like low vswing of
eDP1.4 where the values are different.

Done using following cocci patch for each define:
@@
@@

 # define DP_TRAIN_VOLTAGE_SWING_400     (0 << 0)
+ # define DP_TRAIN_VOLTAGE_SWING_LEVEL_0     (0 << 0)

...

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>