msm: kgsl: remove readl/writel use for dma memory

For dma_alloc_coherent() you don't need writel/readl because
it's just a plain old void *. Linux tries very hard to make a
distinction between io memory (void __iomem *) and memory
(void *) so that drivers are portable to architectures that
don't have a way to access registers via pointer dereferences.
You can see http://lwn.net/Articles/102232/ and the Linus rant
http://lwn.net/Articles/102240/

 here for more details behind
the motivation.

Change-Id: I3da075c30304e4adf321cfb3edb1baa4a93fc2ce
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>