drm/dp: Clarify automated test constant and add constant for FAUX test pattern

  - DP_TEST_LINK_PATTERN is ambiguous, rename to DP_TEST_LINK_VIDEO_PATTERN to clarify
  - Added DP_TEST_LINK_FAUX_PATTERN to support automated testing of Fast AUX

Signed-off-by: Todd Previte <tprevite@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>