drm/i915: Remove useless casts to intel_plane_state

The visible member used to be in intel_plane_state->visible,
but has been moved to drm_plane_state->visible. In the conversion
some casts were left in that are now useless.

to_intel_plane_state(x)->base.visible is the same as x->visible,
so use the latter to clear up the code a little.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484214225-30328-1-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>