PCI: rockchip: Factor out rockchip_pcie_disable_clocks()

Factor out rockchip_pcie_disable_clocks() so it can be reused by other
functions.

No functional change intended, but it does change the order of unpreparing
clocks in the rockchip_pcie_resume_noirq() error path so it matches the
other paths.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>