drm/i915/crt: fix DPMS standby and suspend mode handling

PCH platforms and VLV should map DPMS standby and suspend modes to off, but
due to a buggy reversed comparison this is done on pre-PCH platforms
instead.

Reported-by: Loïc Yhuel <loic.yhuel@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56754


Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>