drm/amd/pm: optimize the amdgpu_pm_compute_clocks() implementations

Drop cross callings and multi-function APIs. Also avoid exposing
internal implementations details.

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