drm/amd/display: Fix LTTPR not Enabled

drm/amd/display: Fix LTTPR not Enabled

[WHY]
Previous LTTPR change has caused a regression that led to an
issue where LTTPR is disabled

[HOW]
Extended changes from previous fix to DCN30X

Reviewed-by: Wesley Chalmers <wesley.chalmers@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Angus Wang <angus.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>