drm/i915/dp_link_training: Disallow autoretrains after failed modeset

intel_dp_check_link_state() and intel_dp_link_params_valid() perform
only a coarse validation of the link configuration used by the active
mode against the available configurations (as constrained by the link
training fallback code after a previous LT failure). Even if these
coarse checks find a seemingly usable configuration, the modeset check,
which performs full verification, may still fail.

Disallow further autoretrain attempts if an autoretrain modeset fails.
Further attempts would just reuse the same modeset parameters and fail
in the same way. Autoretrain will be reallowed unconditionally when the
sink reports a change in its capabilities. This allows an autoretrain to
proceed once both the link validation and modeset checks confirm a
usable configuration.

Also clarify in intel_dp_check_link_state() and
intel_dp_link_params_valid() that these checks are coarse and that a
full validation is only performed by the subsequent atomic modeset
check.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260601093836.3057345-21-imre.deak@intel.com

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