drm/i915: Update legacy primary state outside the commit hook, v2.

This should allow not running plane commit when the crtc is off.
While the atomic helpers update those, crtc->x/y is only updated
during modesets, and primary plane is updated after this function
returns.

Unfortunately non-atomic watermarks and fbc still depend on this
state inside i915, so it has to be kept in sync.

Changes since v1:
- Add comment that the legacy state is updated for fbc.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>