drm/i915: Remove Braswell GGTT update w/a

Testing with concurrent GGTT accesses no longer show the coherency
problems from yonder, commit 5bab6f60 ("drm/i915: Serialise updates
to GGTT with access through GGTT on Braswell"). My presumption is that
the root cause was more likely fixed by commit 3b5724d7 ("drm/i915:
Wait for writes through the GTT to land before reading back"), along
with the use of WC updates to the global gTT in commit 8448661d


("drm/i915: Convert clflushed pagetables over to WC maps". Given
that the original symptoms can no longer be reproduced, time to remove
the workaround.

Testcase: igt/gem_concurrent_blit
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170220124718.14796-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>