drm/amd/powerplay: clean up the APIs for bootup clocks

Combine and simplify the logics for retrieving bootup
clocks.

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>