drm/doc: Reorg for drm-kms.rst

- Again adjust headings a bit, and don't mix up the initialization
  sections with other stuff.
- Remove the doc for output polling, that vfunc is now properly
  documented in the vfunc reference sections.
- Move the grab-bag with all the core stuff (i.e. drm_crtc.[hc]) to
  the front for a more prominent place.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-5-git-send-email-daniel.vetter@ffwll.ch