drm/i915: Extract intel_ddi_post_disable_hdmi_or_sst()

Extract the "not-MST" stuff from intel_ddi_post_disable() so that
the whole thing isn't so cluttered.

The bigjoiner slave handling was outside of the !MST check,
but it really should have been inside it as its the counterpart
to the master handling inside the check. So we pull that
in as well. There is no functional change here as we don't
currently support bigjoiner+MST anyway.

v2: Rebase
v3: Actually extract the slave code as claimed in
    the commit message (presumably a rebase fail in v2)

Tested-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409163502.29633-2-ville.syrjala@linux.intel.com