drm/dp: Add LTTPR helpers

Add the helpers and register definitions needed to read out the common
and per-PHY LTTPR capabilities and perform link training in the LTTPR
non-transparent mode.

v2:
- Add drm_dp_dpcd_read_phy_link_status() and DP_PHY_LTTPR() here instead
  of adding these to i915. (Ville)
v3:
- Use memmove() to convert LTTPR to DPRX link status format. (Ville)

Cc: dri-devel@lists.freedesktop.org
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007170917.1764556-5-imre.deak@intel.com