drm: Handle properties in the core for atomic drivers

The reason behind the original indirection through the helper
functions was to allow existing drivers to overwrite how they handle
properties. For example when a vendor-specific userspace had
expectations that didn't match atomic. That seemed likely, since
atomic is standardizing a _lot_ more of the behaviour of a kms driver.

But 20 drivers later there's no such need at all. Worse, this forces
all drivers to hook up the default behaviour, breaking userspace if
they forget to do that. And it forces us to export a bunch of core
function just for those helpers.

And finally, these helpers are the last places using
drm_atomic_legacy_backoff() and the implicit acquire_ctx.

This patch here just implements the new behaviour and updates the
docs. Follow-up patches will garbage-collect all the dead code.

v2: Fixup docs even better!

v3: Make it actually work ...

v4: Drop the uses_atomic_modeset() checks from the previous patch
again, since they're now moved up in the callchain.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org> (v3)
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170725120204.2107-1-daniel.vetter@ffwll.ch