According to BSpec the Data Island Packet should be disabled after
disabling the transcoder, but before the transcoder clock select is set
to none. On an ICL RVP, daisy-chained MST config not following this
leads to a hang with the following MCE when disabling the output:

[  870.948739] mce: [Hardware Error]: CPU 0: Machine Check Exception: 5 Bank 6: ba00000011000402
[  871.019212] mce: [Hardware Error]: RIP !INEXACT! 10:<ffffffff81aca652> {poll_idle+0x92/0xb0}
[  871.019212] mce: [Hardware Error]: TSC 135a261fe61
[  871.019212] mce: [Hardware Error]: PROCESSOR 0:706e5 TIME 1591739604 SOCKET 0 APIC 0 microcode 20
[  871.019212] mce: [Hardware Error]: Run the above through 'mcelog --ascii'
[  871.019212] mce: [Hardware Error]: Machine check: Processor context corrupt
[  871.019212] Kernel panic - not syncing: Fatal machine check
[  871.019212] Kernel Offset: disabled

Bspec: 4287

Fixes: fa37a213 ("drm/i915: Stop sending DP SDPs on ddi disable")
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200609220616.6015-1-imre.deak@intel.com