drm/ttm: factor out ttm_bo_mmap_vma_setup

Factor out ttm vma setup to a new function.
Reduces code duplication a bit.

v2: don't change vm_flags (moved to separate patch).
v4: make ttm_bo_mmap_vma_setup static.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191016115203.20095-7-kraxel@redhat.com