gpu: drm: atomic_helper: Fix spelling errors

Trivial fixes identified while working on the DRM code.

  s/artifically/artificially/
  s/achive/achieve/

v2: - Actually spell achieve correctly!

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190312003307.14116-1-kieran.bingham+renesas@ideasonboard.com