drm/doc: Put all driver docs into a separate chapter

We have quite a few driver docs now, which is great, but having them
all in the top-level gpu documentation chapter makes it harder to spot
the core/shared bits.

Stuff them into a separate chapter and ecourage people to add even
more!

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180316075926.13584-1-daniel.vetter@ffwll.ch