drm/amd/display: Initialize lt_settings on instantiation

[WHY]
lt_settings' pointers remain uninitialized but nonzero if display fails
to light up with no DPCD/EDID info populated, leading to a hang on access

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@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>