drm/amd/display: Improvements in secure display

[Why]
- Need error message when failing to allocating secure_display_ctx.
- Need to check if secure display context in psp is initialized or not
before using it.

[How]
- Add error message when memory allocation fail.
- Add check before accessing psp secure display context.

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