drm/amd/display: BIOS LTTPR Caps Interface

[WHY]
Some platforms will have LTTPR capabilities forced on by VBIOS flags;
the functions added here will access those flags.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>