drm/radeon: Add the missed drm_gem_object_put() in radeon_user_framebuffer_create()

radeon_user_framebuffer_create() misses to call drm_gem_object_put() in
an error path. Add the missed function call to fix it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org