drm/amd/display: use a more appropriate return value in dp_retrieve_lttpr_cap()

Not all ASICs support LTTPR, however if they don't it doesn't mean that
we have encountered unexpected behaviour. So, use DC_NOT_SUPPORTED
instead of DC_ERROR_UNEXPECTED.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>