drm/amd/display: Fix for null pointer access for ddc pin and aux engine.

[Why]
Need a check for NULL pointer access for ddc pin and aux engine.

[How]
Adding a check for ddc pin and aux engine accesses.

Reviewed-by: Jimmy Kizito <jimmy.kizito@amd.com>
Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>