drm/i915: Treat fb->offsets[] as a raw byte offset instead of a linear offset

Userspace wants to treat fb->offsets[] as raw byte offsets into the gem
bo. Adjust the kernel code to match.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170824191100.10949-2-ville.syrjala@linux.intel.com


Acked-by: Ben Widawsky <ben@bwidawsk.net>
Fixes: 2e2adb05 ("drm/i915: Add render decompression support")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>