drm/shmem-helper: Add missing vunmap on error

The vmapping of dma-buf may succeed, but DRM SHMEM rejects the IOMEM
mapping, 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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220630200058.1883506-2-dmitry.osipenko@collabora.com