[WiP] drm/exynos: Further WiP on S3C6410 G3D driver

Changes done:
 - use separate wait queues for idle and normal flushing to avoid
   spurious wake-ups,
 - defer request retirement until some work is dispatched to GPU to get
   better parallelism and fix too early retirement of state that could
   be still used by GPU,
 - remove unused stall_mutex,
 - rename g3d_update_state() to g3d_prepare_draw() and call it directly
   from g3d_handle_draw_buffer() to simplify the code.