drm/amd/display: resolve minor log problems

[Why]
dc_add_stream_to_context is used to check bw requirement.
It is not an error if it fails.

[How]
Replace DC_ERROR with DC_LOG_WARNING.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>