drm: add generic zpos property

version 8:
- move drm_blend.o from drm-y to drm_kms_helper-y to avoid
  EXPORT_SYMBOL(drm_atomic_helper_normalize_zpos)
- remove dead function declarations in drm_crtc.h

version 7:
- remove useless EXPORT_SYMBOL()
- better z-order wording in Documentation

version 6:
- add zpos in gpu documentation file
- merge Ville patch about zpos initial value and API improvement.
  I have split Ville patch between zpos core and drivers

version 5:
- remove zpos range check and comeback to 0 to N-1
  normalization algorithm

version 4:
- make sure that normalized zpos value is stay
  in the defined property range and warn user if not

This patch adds support for generic plane's zpos property property with
well-defined semantics:
- added zpos properties to plane and plane state structures
- added helpers for normalizing zpos properties of given set of planes
- well defined semantics: planes are sorted by zpos values and then plane
  id value if zpos equals

Normalized zpos values are calculated automatically when generic
muttable zpos property has been initialized. Drivers can simply use
plane_state->normalized_zpos in their atomic_check and/or plane_update
callbacks without any additional calls to DRM core.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

Compare to Marek's original patch zpos property is now specific to each
plane and no more to the core.
Normalize function take care of the range of per plane defined range
before set normalized_zpos.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Cc: Inki Dae <inki.dae@samsung.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: vincent.abriou@st.com
Cc: fabien.dessenne@st.com
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>