msm: kgsl: Write a cookie into HFI write buffer remainder

HFI commands can be any number of dwords in length. But commands
are required to start at 4 dword alignemnt. Instead of skipping
any remainder dwords at the end of a command (and possibly leaving
junk in them), write a cookie into the unused dwords up to the
next alignment.

Change-Id: I12f3f1c37aa09fe57f08f6a1d0bd2cf122e93ee1
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>