drm/i915: Convert BUG_ON(pin_count) from an impossible condition
drm/i915: Convert BUG_ON(pin_count) from an impossible condition

Also spotted by Dan Carpenter.

obj->pin_count is unsigned so the BUG_ON(obj->pin_count<0) will never
trigger.

Signed-off-by: Chris Wilson <711c73f64afdce07b7e38039a96d2224209e9a6c@chris-wilson.co.uk>

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