drm/amd/display: Add null check to dc_submit_i2c_oem

[why]
dc_submit_i2c_oem could be called with ddc null

[how]
add null check and fail the call instead

Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Martin Leung <martin.leung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>