drm/i915/bxt: Sanitize DC state tracking during system resume

After suspend-to-ram or -disk we don't know what power state the display
HW will be, DC0 or DC9 are both possible states, so reset the software
DC state tracking in these cases. This gets rid of 'DC state mismatch'
error messages during resuming from ram or disk where we expected to be
in DC9 (as set by the suspend handler) but we are in DC0.

v2:
- Remove extra WS in gen9_sanitize_dc_state() (Bob)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461173277-16090-4-git-send-email-imre.deak@intel.com