drm/i915: Rename the plane_state->main/aux to plane_state->color_plane[]

Make the main/aux surface stuff a bit more generic by using an array
of structures. This will allow us to deal with both the main and aux
surfaces with common code.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907152413.15761-5-ville.syrjala@linux.intel.com