drm/i915: Polish intel_atomic_track_fbs()

Streamline the code a bit by using intel_ types instead of drm_
types in intel_atomic_track_fbs().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190701160550.24205-4-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>