drm/amd/display: add a NULL pointer check

[Why & How]

We have to check if stream is properly initialized before calling
find_matching_pll(), otherwise we might end up trying to deferecence a
NULL pointer.

Cc: stable@vger.kernel.org # 6.1+
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Sung-huai Wang <danny.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>