drm/i915: Use plane_name() in gen7_enable_fbc()

All the other .enable_fbc() funcs use plane_name(). Make
gen7_enable_fbc() do the same.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>