drm/i915: Reenable LTTPR non-transparent LT mode for DPCD_REV<1.4

drm/i915: Reenable LTTPR non-transparent LT mode for DPCD_REV<1.4

The driver currently disables the LTTPR non-transparent link training
mode for sinks with a DPCD_REV<1.4, based on the following description
of the LTTPR DPCD register range in DP standard 2.0 (at the 0xF0000
register description):

""
LTTPR-related registers at DPCD Addresses F0000h through F02FFh are valid
only for DPCD r1.4 (or higher).
"""

The transparent link training mode should still work fine, however the
implementation for this in some retimer FWs seems to be broken, see the
References: link below.

After discussions with DP standard authors the above "DPCD r1.4" does
not refer to the DPCD revision (stored in the DPCD_REV reg at 0x00000),
rather to the "LTTPR field data structure revision" stored in the
0xF0000 reg. An update request has been filed at vesa.org (see
wg/Link/documentComment/3746) for the upcoming v2.1 specification to
clarify the above description along the following lines:

"""
LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
valid only for LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV 1.4 (or
higher)
"""

Based on my tests Windows uses the non-transparent link training mode
for DPCD_REV==1.2 sinks as well (so presumably for all DPCD_REVs), and
forcing it to use transparent mode on ICL/TGL platforms leads to the
same LT failure as reported at the References: link.

Based on the above let's assume that the transparent link training mode
is not well tested/supported and align the code to the correct
interpretation of what the r1.4 version refers to.

Reported-and-tested-by: Casey Harkins <caseyharkins@gmail.com>
Tested-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
References: https://gitlab.freedesktop.org/drm/intel/-/issues/3415
Fixes: 264613b406eb ("drm/i915: Disable LTTPR support when the DPCD rev < 1.4")
Cc: <stable@vger.kernel.org> # v5.11+
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210512212809.1234701-1-imre.deak@intel.com