drm/i915/hsw: Drop the stereo 3D enabled check in psr_compute_config()

We should not access hardware while computing config also we don't
support stereo 3D so this test was never true.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121225441.18785-6-jose.souza@intel.com