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.