drm/dp: Add DP_DPCD_REV_XX to drm_dp_helper

As more differentation occurs between DP spec. Its useful to have these
as macros in a drm_dp_helper.

v2: DPCD_REV_XX to DP_DPCD_REV_XX

Signed-off-by: Matt Atwood <matthew.s.atwood@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/20180504221800.17830-1-matthew.s.atwood@intel.com