drm/i915: context switch implementation

Implement the context switch code as well as the interfaces to do the
context switch. This patch also doesn't match 1:1 with the RFC patches.
The main difference is that from Daniel's responses the last context
object is now stored instead of the last context. This aids in allows us
to free the context data structure, and context object independently.

There is room for optimization: this code will pin the context object
until the next context is active. The optimal way to do it is to
actually pin the object, move it to the active list, do the context
switch, and then unpin it. This allows the eviction code to actually
evict the context object if needed.

The context switch code is missing workarounds, they will be implemented
in future patches.

v2: actually do obj->dirty=1 in switch (daniel)
Modified comment around above
Remove flags to context switch (daniel)
Move mi_set_context code to i915_gem_context.c (daniel)
Remove seqno , use lazy request instead (daniel)

v3: use i915_gem_request_next_seqno instead of
      outstanding_lazy_request (Daniel)
remove id's from trace events (Daniel)
Put the context BO in the instruction domain (Daniel)
Don't unref the BO is context switch fails (Chris)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -117,6 +117,7 @@ struct  intel_ring_buffer {
 	wait_queue_head_t irq_queue;
 
 	struct i915_hw_context *default_context;
+	struct drm_i915_gem_object *last_context_obj;
 
 	void *private;
 };