drm/i915: Use alphabetical names for sprites

Add sprite_name() macro which should be used with the kind of sprites
that are fixed to pipes (gen4.5+).

Also use dev_priv->num_plane to calculate the sprite index insted
assuming two sprites per pipe. This should make it print the right
name.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>