drm/i915/display/icl: In port sync mode disable slaves first then master

In the transcoder port sync mode, the slave transcoders mask their vblanks
until master transcoder's vblank so while disabling them, make
sure slaves are disabled first and then the masters.

v5:
* Dont pass dev priv to get_slave_crtc (Ville)
v4:
* Obtain slave state from master (Maarten)
v3:
* Rebase
v2:
* Use the intel_old_crtc_state_disables() helper

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191018172725.1338-6-manasi.d.navare@intel.com