drm/i915: possibly invalidate TLB before context switch

From http://intellinuxgraphics.org/documentation/SNB/IHD_OS_Vol1_Part3.pdf

[DevSNB] If Flush TLB invalidation Mode is enabled it's the driver's
responsibility to invalidate the TLBs at least once after the previous
context switch after any GTT mappings changed (including new GTT
entries).  This can be done by a pipelined PIPE_CONTROL with TLB inv bit
set immediately before MI_SET_CONTEXT.

On GEN7 the invalidation mode is explicitly set, but this appears to be
lacking for GEN6. Since I don't know the history on this, I've decided
to dynamically read the value at ring init time, and use that value
throughout.

v2: better comment (daniel)

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

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -116,6 +116,10 @@ struct  intel_ring_buffer {
 
 	wait_queue_head_t irq_queue;
 
+	/**
+	 * Do an explicit TLB flush before MI_SET_CONTEXT
+	 */
+	bool itlb_before_ctx_switch;
 	struct i915_hw_context *default_context;
 	struct drm_i915_gem_object *last_context_obj;