drm/amd/display: Skip plane when not found by stream id

drm/amd/display: Skip plane when not found by stream id

[Why & How]
dml_stream_idx will be -1 when it is not found. Check and skip in such a
case as -1 is not a valid array index.

This fixes a NEGATIVE_RETURNS issue reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>