drm/i915: Change use get_new_plane_state instead of existing plane state

The get_existing macros are deprecated and should be replaced by
get_old/new_state for clarity.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409124656.39886-1-maarten.lankhorst@linux.intel.com
[mlankhorst: Remove useless warn. (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>