drm/amd/display: Skip accessing array for unknown eng_id

[WHY]
ENGINE_ID_UNKNOWN (-1) is not a valid eng_id and not a valid array
index.

[HOW]
Check whether eng_id is unknown to avoid access array with negative
array index.

This fixes 4 OVERRUN issues 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>