Right now dp.regs.dp_tp_ctl/status are only set during the encoder
pre_enable() hook, what is causing all reads and writes to those
registers to go to offset 0x0 before pre_enable() is executed.

So if i915 takes the BIOS state and don't do a modeset any following
link retraing will fail.

In the case that i915 needs to do a modeset, the DDI disable sequence
will write to a wrong register not disabling DP 'Transport Enable' in
DP_TP_CTL, making a HDMI modeset in the same port/transcoder to
not light up the monitor.

So here for GENs older than 12, that have those registers fixed at
port offset range it is loading at encoder/port init while for GEN12
it will keep setting it at encoder pre_enable() and during HW state
readout.

Fixes: 4444df6e ("drm/i915/tgl: move DP_TP_* to transcoder")
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200414230442.262092-1-jose.souza@intel.com
(cherry picked from commit edcb9028)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>