drm/i915: atomic plane updates in a nutshell

Now that all planes are added during a modeset we can use the
calculated changes before disabling a plane, and then either commit
or force disable a plane before disabling the crtc.

The code is shared with atomic_begin/flush, except watermark updating
and vblank evasion are not used.

This is needed for proper atomic suspend/resume support.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90868


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>