drm/i915: Fix sprite_scaling_enabled for multiple sprites

We have more than one sprite, so a boolean simply won't cut it.
Turn sprite_scaling_enabled into a bitmask and track the state
of sprite scaler for each sprite independently.

Also don't re-enable LP watermarks until the sprite registers
have actually been written, and thus sprite scaling has really
been disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>