msm: kgsl: create sysfs entries to expose memory usage

Added sysfs entries to show kgsl memory usage statistics.
gpumem_mapped: kgsl memory mapped in the process address space.
gpumem_unmapped: kgsl allocated memory but not mapped in process.
imported_mem: graphics memory not allocated by the kgsl.

Below is the sysfs path for new entries:
/sys/class/kgsl/kgsl/proc/<pid>/

Change-Id: I08c2014d28dc0ca1e2b54ebf966d00143b303b54
Signed-off-by: Amit Kushwaha <kushwaha@codeaurora.org>