drm/i915/ddi: Don't frob the DP link scramble disabling flag

drm/i915/ddi: Don't frob the DP link scramble disabling flag

According to BSpec this flag should not be changed while the DDI
function is enabled. On BDW+ the DP_TP_CTL register spec also states it
explicitly that the HW takes care of enabling/disabling the scrambling
for training patterns (and it must stay enabled for normal pixel
output). Assume that this HW automatic handling of scrambling is also
true for HSW.

BSpec: 8013, 7557, 50484

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200714153141.10280-1-imre.deak@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>