drm/doc: Reorganize driver documentation

Split up the DocBook into the core drm part and a 2nd part for
driver documentation. As an example add a very (very!) basic
skeleton for i915.

v1: Typo fixes from Dieter.

Cc: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>