msm: kgsl: ioremap rgmu with devm_ioremap

We can't use devm_ioremap_resource here because
we purposely double map the gpu_cc registers for
debugging purposes.

Change-Id: If6e60431fe54f9d647fd09e5e633d7b7edc3c62f
Signed-off-by: Hui Li <hul@codeaurora.org>