drm/i915: Assert the dbuf is enabled when disabling DC5/6

Like with cdclk, the DMC is supposed to manage dbuf enabling/disabling.
Let's make sure it has correctly restored the dbuf state to enabled
when we disable the DC states.

v2: s/skl/gen9/ in function name (Imre)

Cc: Imre Deak <imre.deak@intel.com>
Suggested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463407180-28993-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>