drm/i915: Use enum plane_id in VLV/CHV sprite code

Use intel_plane->id to derive the VLV/CHV sprite register offsets
instead of abusing plane->plane which is really meant to for
primary planes only.

v2: Convert assert_sprites_disabled() over as well
v3: Rename the reg macro parameter to 'plane_id' as well (Paulo)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479830524-7882-6-git-send-email-ville.syrjala@linux.intel.com