drm/amd/display: num of sw i2c/aux engines less than num of connectors

[why]
AMD Stoney reference board, there are only 2 pipes (not include
underlay), and 3 connectors. resource creation, only
2 I2C/AUX engines are created. Within dc_link_aux_transfer, when
pin_data_en =2, refer to enengines[ddc_pin->pin_data->en] = NULL.
NULL point is referred later causing system crash.

[how]
each asic design has fixed number of ddc engines at hw side.
for each ddc engine, create its i2x/aux engine at sw side.

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>