drm/i915: Add an atomic evasion step to watermark programming, v4.

Allow the driver to write watermarks during atomic evasion.
This will make it possible to write the watermarks in a cleaner
way on gen9+.

intel_atomic_state is not used here yet, but will be used when
we program all watermarks as a separate step during evasion.

This also writes linetime all the time, while before it was only
done during plane updates. This looks like this could be a bugfix,
but I'm not sure what it affects.

Changes since v1:
- Add comment about atomic evasion to commit message.
- Unwrap I915_WRITE call. (Lyude)
Changes since v2:
- Rename atomic_evade_watermarks to atomic_update_watermarks. (Ville)
- Add line wraps where appropriate, fix grammar in commit message. (Matt)
Changes since v3:
- Actually fix commit message. (Matt)
- Line wrap calls to watermark update functions. (Matt)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1478609742-13603-2-git-send-email-maarten.lankhorst@linux.intel.com