msm: pcie: switch pipe clk mux source to XO before disabling GDSC

Having pipe clk mux source set to PCIe pipe clk can possibly
cause PCIe gdsc to be stuck while being disabled until the
source is switched to XO. Therefore switch the source to XO
before disabling PCIe gdsc.

Change-Id: I5292c1e3b7a006b3857ef82fa446392d324d616f
Signed-off-by: Tony Truong <truong@codeaurora.org>