drm/i915: Fix rsvd2 mask when out-fence is returned

GENMASK_ULL wants the high bit of the mask first. The current value
cancels the in-fence when an out-fence is returned.

Fixes: fec0445c

 ("drm/i915: Support explicit fencing for execbuf")
Testcase: igt/gem_exec_fence/keep-in-fence*
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214191827.8465-1-daniele.ceraolospurio@intel.com
Cc: <stable@vger.kernel.org> # v4.12+