drm/radeon: use prime helpers

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

v2:
- Rename functions to radeon_gem_prime_get_sg_table and
  radeon_gem_prime_import_sg_table.
- Delete the now-unused vmapping_count variable.

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>