drm/amd/display: Fix the NULL pointer. (v2)

ret value in amdgpu_dm_atomic_check was not rest to EINVAL
after drm_atomic_add_affected_planes and by this making
any subsequent validation failure pass when returning
to atomic_check.

v2: Add WARN_ON print for dc_commit_streams in
amdgpu_dm_atomic_commit_tail since this should never
fail.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>