drm/radeon: remove dead function def

Was removed in the async VM update series.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_asic.h

@@ -440,7 +440,6 @@ int cayman_asic_reset(struct radeon_device *rdev);
 void cayman_ring_ib_execute(struct radeon_device *rdev, struct radeon_ib *ib);
 int cayman_vm_init(struct radeon_device *rdev);
 void cayman_vm_fini(struct radeon_device *rdev);
-void cayman_vm_unbind(struct radeon_device *rdev, struct radeon_vm *vm);
 void cayman_vm_flush(struct radeon_device *rdev, struct radeon_ib *ib);
 uint32_t cayman_vm_page_flags(struct radeon_device *rdev, uint32_t flags);
 void cayman_vm_set_page(struct radeon_device *rdev, struct radeon_vm *vm,