drm/i915: s/int plane/int color_plane/

To reduce the confusion between a drm plane and the planes of
framebuffers let's desiginate the latter as "color plane".

Weak-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/20180907152413.15761-8-ville.syrjala@linux.intel.com