On device removal reroute all CPU mappings to dummy page.

v3:
Remove loop to find DRM file and instead access it
by vma->vm_file->private_data. Move dummy page installation
into a separate function.

v4:
Map the entire BOs VA space into on demand allocated dummy page
on the first fault for that BO.

v5: Remove duplicate return.

v6: Polish ttm_bo_vm_dummy_page, remove superfluous code.
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210512142648.666476-2-andrey.grodzovsky@amd.com