drm/amdgpu: add an asic callback for pre asic init

drm/amdgpu: add an asic callback for pre asic init

This callback can be used by asics that need to
do something special prior to calling atom asic init.

Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>