drm/amd/display: Set LTTPR Transparent Mode after read link cap

[WHY]
SCR for DP 2.0 Spec states that a DPTX shall put LTTPRs into Transparent
mode after reading LTTPR Capability registers on HPD.

The wording of the SCR is somewhat ambiguous as to whether
Transparent mode must be set explicity, or is implicitly set on LTTPR
capability read. Explicitly setting Transparent mode after LTTPR
capability read should cover all
cases.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>