drm: Fix object leak in DRM_IOCTL_GEM_CHANGE_HANDLE

drm: Fix object leak in DRM_IOCTL_GEM_CHANGE_HANDLE

Add missing drm_gem_object_put() call when drm_gem_object_lookup()
successfully returns an object. This fixes a GEM object reference
leak that can prevent driver modules from unloading when using
prime buffers.

Fixes: 53096728b891 ("drm: Add DRM prime interface to reassign GEM handle")
Cc: <stable@vger.kernel.org> # v6.18+
Signed-off-by: Karol Wachowski <karol.wachowski@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Maciej Falkowski <maciej.falkowski@linux.intel.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20251212134133.475218-1-karol.wachowski@linux.intel.com