msm: kgsl: Keep track of kernel space mappings to memory

Add a counter to count number of times a memory descriptor is
mapped in the kernel and unmap the kernel mapping only when the
count reaches 0. There can be multiple threads using the kernel
mapping at the same time hence a counter is used to make sure
that we do not unmap the mapping before the last thread has
finished using it.

Change-Id: I9c53deb6af12fb1f6ed30e46b386e1cd60dafdda
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>