msm: kgsl: Cleanup the memory free list

Tighten up the memory free list code and remove the memory unused
debugfs file.  Outside of the cleanup the functional changes are
a) move the mutex to a spinlock for speed, and b) only return the
last entry in the list for given pid/gpu address combination.

Change-Id: Ic0dedbad9595302e2b42de2807fa82c425d1717f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>