msm: kgsl: Restructure IOMMU clock management

Restructure IOMMU clock management code to enable/disable clocks
based on iommu unit instead of context. In hardware IOMMU clocks
are associated with each IOMMU unit. There is no granularity of
clock control for each iommu context. Turning on the clocks for
one context in a IOMMU unit automatically turns on the clocks for
all the contexts in IOMMU unit. Reduce the number of clock
enable/disable calls and structure the software design as per
hardware design by doing clock management per IOMMU unit.

Change-Id: Ib54dbb7a31d6b30b65c07ef130bee149e3e587d8
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>