drm/i915/gvt: fix nested sleeping issue

We cannot use blocking method mutex_lock inside a wait loop.
Here we invoke pick_next_workload() which needs acquire a
mutex in our "condition" experssion. Then we go into a another
of the going-to-sleep sequence and changing the task state.
This is a dangerous. Let's rewrite the wait sequence to avoid
nested sleeping.

v2: fix do...while loop exit condition (zhenyu)
v3: rebase to gvt-staging branch

Signed-off-by: Du, Changbin <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>