drm/i915: Move stuff from haswell_crtc_disable() into encoder .post_disable()

Move all of haswell_crtc_disable() into the encoder
.post_disable() hooks. Now we're left with just
calling the .disable() and .post_disable() hooks
back to back.

I chose to move the code into the .post_disable() hook instead
of the .disable() hook as most of the sequence is currently
implemented in the .post_disable() hook.

We should collapse it all down to just one hook and then the
encoders can drive the modeset sequence fully. But that may
need some further refactoring as we currently call the
ddi .post_disable() hook from mst code and we can't just
replace that with a call to the ddi .disable() hook.

Should also follow up with similar treatment for the enable
sequence but let's start here where it's easier.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213195217.15168-5-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>