msm: kgsl: Allocate memory for GMU System Write buffer SWA

While flushing the GMU System Write buffer, two dummy writes must
be posted to it to guarantee that all writes preceding the dummy
writes have been posted to system memory.

Change-Id: I004da3f85294e20dcf6960652cf51a5dba008ef8
Signed-off-by: Sushmita Susheelendra <ssusheel@codeaurora.org>
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>