drm/i915: Also check view->type for a normal GGTT view

We cannot simply use !view as shorthand for all normal GGTT views as a
few callers will always populate a i915_ggtt_view struct and set the
type to NORMAL instead. So check for (!view || view->type == NORMAL)
inside i915_gem_object_ggtt_pin().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220134208.24988-1-chris@chris-wilson.co.uk