drm/amd/powerplay: widely share the API for data table retrieving

Considering the data table retrieving can be more widely shared,
amdgpu_atombios.c is the right place.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>