msm: kgsl: Fix the race between context create and destroy

Hold the context lock before updating the context id in
param->drawctxt_id to avoid race condition between context
creation and context destroy.

Bug: 36491445
Change-Id: Ic26d3e5b68078c02d15c38080b1a262ea4b1f7fe
Signed-off-by: Sunil Khatri <sunilkh@codeaurora.org>