drm/amd/display: Fix DPCD translation for LTTPR AUX_RD_INTERVAL

[Why]
The translation between the DPCD value and the specified AUX_RD_INTERVAL
in the DP spec do not match.

[How]
Update values to match the spec.

Signed-off-by: George Shen <george.shen@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@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>