iommu/msm: Fix "scheduling while atomic" bug

In the msm iommu driver currently we incorrectly use clk_prepare_enable()
and clk_disable_unprepare() functions that are sleeping and should not be
called in atomic context. Fix this by preparing clocks in advance and use
only clk_enable() in atomic context.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>