drm/i915: Remove get_existing_crtc_state

get_existing_crtc_state is currently unused, get rid of it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409124656.39886-2-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>