drm/i915: Fix LP1 sprite watermarks for ILK/SNB

ILK/SNB don't have LP2+ watermarks for sprites. Also the LP1 sprite
watermark register has its own enable bit. Take these differences
into account when programming the LP1+ registers.

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