drm/omap: Rework the rotation-on-crtc hack

I want/need to rework the core property handling, and this hack is
getting in the way. But since it's a non-standard propety only used by
legacy userspace we know that this will only every be called from
ioctl code. And never on some other free-standing state struct, where
this old hack wouldn't work either.

v2: don't forget zorder and get_property!

v3: Shadow the legacy state to avoid locking issues in get_property
(Maarten).

v4: Review from Laurent
- Move struct omap_crtc_state into omap_crtc.c
- Clean up comments.
- Don't forget to copy the shadowed state over on duplicate.

v5: Don't forget to update the reset handler (Maarten).
v6: Update omap_crtc_state shadow values in omap_crtc_atomic_check (Maarten).
v7:
- Fix get_property to return 0 and set value in *val (Maarten).
- Update comment in set_property for changes in v6 (Maarten).

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> (v4)
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> (v4)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/a6a10a4f-2ebc-5f81-00bd-5e906967f384@linux.intel.com