drm/i915: Remove duplicate and unused register #defines in i915_reg.h

TRANS_DP_VIDEO_AUDIO is not used at all.
The other 3 has duplicated #defines.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>