Revert "drm/i915: Enable PSR by default on Valleyview and Cherryview."

This reverts commit a38c274f.

PSR causes all sorts of vblank wait timeouts and whanot on CHV. Disable
it again.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Fixes: a38c274f

 ("drm/i915: Enable PSR by default on Valleyview and Cherryview.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457543247-13987-2-git-send-email-ville.syrjala@linux.intel.com
Link: http://patchwork.freedesktop.org/patch/msgid/1457543247-13987-2-git-send-email-ville.syrjala@linux.intel.com
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>