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.

Cc: stable@vger.kernel.org
Fixes: 22676bc5 ("drm/amd/display: Fix dmub soft hang for PSR 1")
Link: https://github.com/fwupd/fwupd/issues/6327


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>