drm/i915: Don't call intel_plane_restore() when the prop value didn't change

No point in calling intel_plane_restore() in .set_property() if the
value didn't change.

More importantly this papers over a bug where the current primary plane
code forgets to update the user coordinates we store under intel_plane
unless the primary plane .update_plane() hook is actually called. This
means we have 0 in the coordinates straight after boot and any call
to intel_restore_plane() (such as from restore_fbdev_mode()) will
actually turn off the primary plane. This mess needs to be fixed properly
but that's a bigger task and the first step there is killing off
intel_pipe_set_base() and just calling the primary plane
.update_plane() hook. For the immediate problem of black screen after
boot this small patch is enough to hide it.

The problem originates from these two commits:
 commit 3a5f87c2
 Author: Thomas Wood <thomas.wood@intel.com>
 Date:   Wed Aug 20 14:45:00 2014 +0100

    drm: fix plane rotation when restoring fbdev configuration

 commit d91a2cb8e5104233c02bbde539bd4ee455ec12ac
 Author: Sonika Jindal <sonika.jindal@intel.com>
 Date:   Fri Aug 22 14:06:04 2014 +0530

    drm/i915: Add 180 degree primary plane rotation support

Cc: Thomas Wood <thomas.wood@intel.com>
Cc: Sonika Jindal <sonika.jindal@intel.com>
Tested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Tested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>