drm/msm: Increase maximum number of GPU commands

Current default number is 4. It's verified sufficient in power lift,
GFXBench and Conformance tests. But it's reported that in a new
application, Car Renderer, more than 4 commands are submitted to DRM.

Change-Id: Ib418775dcccac1f304a2d6d074b9f82729c957b6
Signed-off-by: Kasin Li <donglil@codeaurora.org>