drm/i915/ringbuffer: Fix sign of ring space.
drm/i915/ringbuffer: Fix sign of ring space.

As we presume space is signed when computing and looking for wrap along,
make it so.

Reported-by: Owain G. Ainsworth <e190ce51278d4af541c8bae1003acd81f77f643d@googlemail.com>
Signed-off-by: Chris Wilson <711c73f64afdce07b7e38039a96d2224209e9a6c@chris-wilson.co.uk>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas