drm/amd/display: fix a NULL pointer dereference in amdgpu_dm_i2c_xfer()

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: fix a NULL pointer dereference in amdgpu_dm_i2c_xfer() When ddc_service_construct() is called, it explicitly checks both the link type and whether there is something on the link which will dictate whether the pin is marked as hw_supported. If the pin isn't set or the link is not set (such as from unloading/reloading amdgpu in an IGT test) then fail the amdgpu_dm_i2c_xfer() call.