msm: kgsl: Add separate GPU shader memory mapping

Add separate kernel memory mapping for GPU shader memory.
Previously, both the register and the shader memory were mapped
as one entity, with the mapping length being equal to combined
size of register memory and shader memory. Now, we separate the
shader memory mapping to help in cases of GPU devices where the
shader and register memory may not be adjacent. This helps to
dump the shader memory in the postmortem snapshot. By having a
separate mapping for shader memory, the snapshot dump routine
simply reads in the shader memory range specified and dumps.

Change-Id: I30e59dffe849fefcfbbaf0e5245cc1024ccedcb1
Signed-off-by: Harsh Vardhan Dwivedi <hdwivedi@codeaurora.org>