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

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 5a35e99e8162d6820013a56ad15ea8bf937af5a6
Author: Damien Lespiau <[email protected]>
Date:   Fri Oct 26 18:20:12 2012 +0100

    drm/i915: adjust sprite base address

Signed-off-by: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Damien Lespiau <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>