PCI: rockchip: Factor out rockchip_pcie_enable_clocks()

Factor out rockchip_pcie_enable_clocks() so it can be reused by
rockchip_pcie_resume_noirq() and rockchip_pcie_probe().

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

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