drm/i915/icl: Do not change reserved registers related to PSR2

For ICL the bit 12 of CHICKEN_TRANS is reserved so we should not
touch it and as by default VSC_DATA_SEL_SOFTWARE_CONTROL is already
unset in gen10 + GLK we can just drop it and fix for both gens.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@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/20181204003403.23361-5-jose.souza@intel.com