drm/amd/display: fix dtm unloading

there was a type in the terminate command.

We should be calling psp_dtm_unload() instead of psp_hdcp_unload()

Fixes: 143f2305 ("drm/amdgpu: psp DTM init")
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>