drm/amdgpu: add a wrapper for atom asic_init

This allows us to add asic specific workarounds for atom
asic init while keeping the adev specifics out of the
atombios parser code.

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