drm/i915: Remove last references to drm_atomic_get_existing* macros

drm/i915: Remove last references to drm_atomic_get_existing* macros

All the references to get_existing_state can be converted to
get_new_state or get_old_state, which means that i915 is now
get_existing_state free.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409124656.39886-3-maarten.lankhorst@linux.intel.com
[mlankhorst: Fix alignment in prepare_plane_fb. (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>