drm/amd/display: Adjust secure_display_context data structure

[Why]
Variables relates to secure display are spreading out within struct
amdgpu_display_manager.

[How]
Encapsulate relevant variables into struct secure_display_context and
adjust relevant affected codes.

Reviewed-by: HaoPing Liu <haoping.liu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>