drm/i915: Configure GEN6_RP_DOWN_TIMEOUT on CHV

CherryViewA0_iGfx_BIOS_DRIVER_PUNIT_spec_y14w28d5 tells us not to enable
the RP down timeout interrupt, and says that the timeout value is hence
not used. We do enable that interrupt currently though, so leaving the
timeout as 0 results in very poor performance as the GPU frequency keeps
dropping constantly. So just program the register with the recommended
value.

Leaving the interrupt enabled doesn't seem to do any harm so far. So
I've decided to leave it on for now, just to avoid making CHV a
special case.

This fixes the performance regression from:
 commit 5a0afd4b
 Author: Deepak S <deepak.s@linux.intel.com>
 Date:   Sat Dec 13 11:43:27 2014 +0530

    drm/i915/chv: Use timeout mode for RC6 on chv

Cc: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak <S&lt;deepak.s@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>