drm/amd/display: Add feature flags to disable LTTPR

[Why]
Allow for disabling non transparent mode of LTTPR for running tests.

[How]
Add a feature flag and set them during init sequence. The flags are
already being used in DC.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>