drm/amd/display: Fix null pointer dereference in error message

This patch fixes a null pointer dereference in the error message that is
printed when the Display Core (DC) fails to initialize. The original
message includes the DC version number, which is undefined if the DC is
not initialized.

Fixes: 9788d087 ("drm/amd/display: improve the message printed when loading DC")
Signed-off-by: Cong Liu <liucong2@kylinos.cn>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>