drm/admgpu: fix dereferencing null pointer context

[ Upstream commit 030ffd4d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>