drm/admgpu: fix dereferencing null pointer context

When user space sets an invalid ta type, the pointer context will be empty.
So it need to check the pointer context before using it

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>