drm/amdgpu: fix potential NULL pointer when check_atom_bios() fails

Asic funcs pointer needs being checked whether is NULL. Because when
check_atom_bios() fails in req_init_data handshake, asic funcs pointer
is not initialized yet.

Signed-off-by: Jiawei Gu <Jiawei.Gu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>