drm/i915: Disable Secure Batches for gen6+

Retroactively stop reporting support for secure batches
through the api for gen6+ so that older binaries trigger
the fallback path instead.

Older binaries use secure batches pre gen6 to access resources
that are not available to normal usermode processes. However,
all known userspace explicitly checks for HAS_SECURE_BATCHES
before relying on the secure batch feature.

Since there are no known binaries relying on this for newer gens
we can kill secure batches from gen6, via I915_PARAM_HAS_SECURE_BATCHES.

v2: rebase (Mika)
v3: rebase (Mika)

Signed-off-by: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris.p.wilson@intel.com>