drm/i915/dp: Don't switch to idle pattern before disable on pre-hsw

drm/i915/dp: Don't switch to idle pattern before disable on pre-hsw

For some reason we are switching over to the idle pattern before
disabling the DP port on pre-hsw. AFAICS this has never been part
of the documented sequence (and on hsw+ the spec explicitly says
not to do this). Get rid of it.

The code goes all the way back to commit 5eb08b69f510 ("drm/i915: enable
DisplayPort support on IGDNG"), and it was accompanied by a 17ms delay
which got changed to vbl wait in commit ab527efc2fea ("drm/i915: use
wait_for_vblank instead of msleep(17)"), and was later completely removed
in  commit 93c9c19b3d25 ("drm/i915: remove unexplained vblank wait in
the DP off code").

Smoke tested on g4x/snb/chv.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250710201718.25310-3-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>