drm/i915: Clean up casts to crtc_state in intel_atomic_commit_tail()

Use old/new_intel_crtc_state, and get rid of all the conversion casts
where they don't add anything.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-2-maarten.lankhorst@linux.intel.com