drm/amd/display: Add DC debug option to force LTTPR mode

[WHY]
Useful for external teams debugging LTTPR issues

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>