drm/i915: Enable display WA#1183 from its correct spot

The DMC FW specific part of display WA#1183 is supposed to be enabled
whenever enabling DC5 or DC6, so move it to the DC6 enable function
from the DC6 disable function.

I noticed this after Daniel's patch to remove the unused
skl_disable_dc6() function.

Fixes: 53421c2f ("drm/i915: Apply Display WA #1183 on skl, kbl, and cfl")
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: <stable@vger.kernel.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180419155109.29451-1-imre.deak@intel.com


(cherry picked from commit b49be662)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>