msm: kgsl: Flush the UCHE when context changes

Add commands to flush the GPU UCHE when a new context submits
commands to GPU. The ensures that the new context does not use
stale data present in UCHE.

Change-Id: I123a323be5f3fb9d1f9f96fed5bb68b8d0d27d76
CRs-Fixed: 607976
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>