msm: npu: Disable npu post clocks separately

NPU post clocks are enabled after fw is loaded, so they should be
disabled before fw is unloaded. This change is to separate
npu_disable_core_clocks into npu_disable_core_clocks and
npu_disable_post_pil_clocks.

Change-Id: I81dd23ab4f40e3dc991b9c4404b9ebcef9259451
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>