drm/amdgpu: Fix NULL pointer when ta is missing

Ta is optional, so check if ta firmware is loaded or not.

Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>