drm/i915: Use pixel size for computing linear offsets into a sprite

This fixes an original bug in the sprite code that miscomputed the
source offset into a linear YUV packed framebuffer, that was magnified
into an oops with

commit 5a35e99e
Author: Damien Lespiau <damien.lespiau@intel.com>
Date:   Fri Oct 26 18:20:12 2012 +0100

    drm/i915: adjust sprite base address

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