drm/i915: Forgo last_fence active request tracking

We were using the last_fence to track the last request that used this
vma that might be interpreted by a fence register and forced ourselves
to wait for this request before modifying any fence register that
overlapped our vma. Due to requirement that we need to track any XY_BLT
command, linear or tiled, this in effect meant that we have to track the
vma for its active lifespan anyway, so we can forgo the explicit
last_fence tracking and just use the whole vma->active.

Another solution would be to pipeline the register updates, and would
help resolve some long running stalls for gen3 (but only gen 2 and 3!)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812174804.26180-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_vma.h

@@ -111,7 +111,6 @@ struct i915_vma {
 #define I915_VMA_GGTT_WRITE	BIT(14)
 
 	struct i915_active active;
-	struct i915_active_request last_fence;
 
 	/**
 	 * Support different GGTT views into the same object.