drm/i915: Fix sprite offset on HSW

HSW consolidates SPRTILEOFF and SPRLINOFF into a single SPROFFSET
register.

v2: Remove a useless level of indentation (Paulo Zanoni)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>