drm/i915: gmch: set SR WMs to valid values before enabling them

Atm it's possible that we enable the memory self-refresh mode before the
watermark levels used by this mode are programmed with valid values. So
move the enabling after we programmed the WM levels.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Reviewed-by: Deepak <S&lt;deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>