drm/i915: use __packed instead of __attribute__((packed))

Checkpatch tells me

WARNING: __packed is preferred over __attribute__((packed))

so switch over to __packed across the driver before adding new packed
structs.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>