msm: kgsl: Do not switch pagetable if context is detached

This is done to avoid a race condition between a context getting
detached and destroyed before the GPU has executed the pt switch
commands.

CRs-Fixed: 987587
Change-Id: I5c485e41a23b288f27e607b3e3ed5bf66cbad98a
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>