drm/tegra: Move fbdev unmap special case

User framebuffers are created with either bo->pages or bo->vaddr set,
depending on whether or not an IOMMU is present. On the other hand, the
framebuffer created for fbdev emulation has a vaddr mapping made if
bo->pages is set after creation. This is set up in fbdev probe.

Remove the special case unmapping from the general-purpose framebuffer
destroy, and move it to fbdev teardown.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Thierry Reding <treding@nvidia.com>