drm/shmem-helper: Add missing vunmap on error

The vmapping of dma-buf may succeed, but DRM SHMEM rejects the iomem
mappings, and thus, drm_gem_shmem_vmap_locked() should unvmap the iomem
before erroring out.

Cc: stable@vger.kernel.org
Fixes: 49a3f51d ("drm/gem: Use struct dma_buf_map in GEM vmap ops and convert GEM backends")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>