msm: kgsl: Only clear references to GMU clocks when removing

When stopping the GMU, don't set the clock handles to NULL
after disabling them. Only do this when removing the
GMU device.

CRs-Fixed: 2017390
Change-Id: Ia2223f91a1f3e84d1c3cf04f31652bb1c5893f6a
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>