drm/ttm: rename ttm_fbdev_mmap

Rename ttm_fbdev_mmap to ttm_bo_mmap_obj.  Move the vm_pgoff sanity
check to amdgpu_bo_fbdev_mmap (only ttm_fbdev_mmap user in tree).

The ttm_bo_mmap_obj function can now be used to map any buffer object.
This allows to implement &drm_gem_object_funcs.mmap in gem ttm helpers.

v3: patch added to series

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191016115203.20095-8-kraxel@redhat.com