drm/msm/a6xx: Track current ctx by seqno

In theory a context can be destroyed and a new one allocated at the same
address, making the pointer comparision to detect when we don't need to
update the current pagetables invalid.  Instead assign a sequence number
to each context on creation, and use this for the check.

Fixes: 84c31ee1 ("drm/msm/a6xx: Add support for per-instance pagetables")
Signed-off-by: Rob Clark <robdclark@chromium.org>