drm/amd/display: Fix detection of 4 lane for DPALT

[Why]
DPALT detection for B0 PHY has its own set of RDPCSPIPE registers

[How]
Use RDPCSPIPE registers to detect if DPALT lane is 4 lane

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Hansen <Hansen.Dsouza@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org