drm/amd/display: Add null pointer check to is_dig_enabled func

[WHY]
Safeguarding as pointer may be null in diagnostic environment

Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Sung Lee <Sung.Lee@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>