drm/i915/hsw: Disable L3 caching of atomic memory operations.

Otherwise using any atomic memory operation will lock up the GPU due
to a Haswell hardware bug.

v2: Use the _MASKED_BIT_ENABLE macro.  Drop drm parameter definition.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: <stable@vger.kernel.org>
[danvet: Fix checkpatch fail.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>