drm/amd/display: Add NULL check for enabling dp ss

[Why]

The pointer for integrated_info can be NULL which causes the system to
do a null pointer deference and hang on boot.

[How]

Add a check to ensure that integrated_info is not null before enabling
DP ss.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>