msm: kgsl: When switching pagetable use current context ID

When we switch pagetables then the pagetable switch commands
should be executed on behalf of the current active context and
not the context to which we are switching. This is because
until the pagetable switch occurs the GPU will be using the
pagetable of the active context.

CRs-fixed: 574989
Change-Id: I9ca65ea399e39a158dc72adc2c7b661f62f18c93
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>