msm: kgsl: fix format of the rbbm read error message

It is much more useful to get the dword offset of the
failing register than the byte offset because it is
consistent with the #defines and other register
logging in postmortem dump and snapshot.

Change-Id: Ie257475a8e2224729e4d1ac14af13a8027974027
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>