drm/i915: Narration overview on GEM

Add a narration to i915.rst about Intel GEN GPU's: engines,
driver context and relocation. Also do minor reorder to improve
narration.

v5:
  More type fixes.
  Flow bullet list so lines are not too long.

Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
[Joonas: correcting the patch title]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523001957-6427-2-git-send-email-kevin.rogovin@intel.com