drm/ast: Remove unnecessary NULL check in bo_unref
drm/ast: Remove unnecessary NULL check in bo_unref

ttm_bo_unref unconditionally calls kref_put on it's argument, so the
thing can't be NULL without already causing Oopses.

Spotted by coverity.

Reviewed-by: David Herrmann <fb53b2ddb8d141e0cb39d7c67c2f81b6bc2eb0f7@gmail.com>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs