drm/i915: clean up atomic plane check functions, v2.

By passing crtc_state to the check_plane functions a lot of duplicated
code can be removed. There are still some transitional helper calls,
they will be removed later.

Changes since v1:
- Revert state->visible changes.
- Use plane->state->crtc instead of plane->crtc.
- Use drm_atomic_get_existing_crtc_state.

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>