drm/i915: Rename the confusing 'plane_id' to 'color_plane'

A variable whose name is 'plane_id' is expected to be of the
enum plane_id type. In this case we have a raw int, which turns
out to refer to the plane of the framebuffer. Rename the variable
to 'color_plane' in line with the trend started earlier.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-13-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>