drm/nouveau: use prime helpers

Simplify the Nouveau prime implementation by using the default behavior provided
by drm_gem_prime_import and drm_gem_prime_export.

v2: Rename functions to nouveau_gem_prime_get_sg_table and
nouveau_gem_prime_import_sg_table.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Dave Airlie <airlied@redhat.com>