drm: Add helper for simple display pipeline

Provides helper functions for drivers that have a simple display
pipeline. Plane, crtc and encoder are collapsed into one entity.

Changes since v4:
- Remove drm_connector_register() call
- Forgot to assign pipe->connector

Changes since v3:
- (struct drm_simple_display_pipe *)->funcs should be const

Changes since v2:
- Drop Kconfig knob DRM_KMS_HELPER
- Expand documentation

Changes since v1:
- Add DOC header and add to gpu.tmpl
- Fix docs: @funcs is optional, "negative error code",
  "This hook is optional."
- Add checks to drm_simple_kms_plane_atomic_check()

Cc: jsarha@ti.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465570559-14238-1-git-send-email-noralf@tronnes.org