drm/i915/bxt: Also add bxt_resume_prepare() to the S3/S4 path

Currently bxt_resume_prepare() is only used in the runtime-resume path.
Add it to the full S3/S4 path as well.

v2: Rebase on top of the vlv_resume_prepare() shuffling around

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>