msm: kgsl: Find a mem_entry by way of a GPU address and a pagetable base

Given a pagetable base and a GPU address, find the struct kgsl_mem_entry
that matches the object.  Move this functionality out from inside another
function and promote it to top level so it can be used by upcoming
functionality.

Change-Id: Ic0dedbad965530a28c43bac36dda0535dcd1c95b
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>