drm/amdgpu/powerplay: get SMC FW size to a flexible way

Get SMC fw size before backdoor loading instead of giving an
certain value, as it may different for different ASIC.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>