msm: kgsl: improve postmortem and cff bounds checking

Some hangs are fooling the postmortem dump code into
running off the end of a buffer. Fix this by making
its bounds check logic work better by reusing the
logic from kgsl_find_region().

CRs-Fixed: 319312
Change-Id: If6cc01369f4b0634d7e19494b8d021643abb8a39
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>