drm/i915/dp: Clear DPCD training pattern before transmitting the idle pattern

We are supposed to switch off the training pattern in DPCD before
we start transmitting the idle pattern. For LTTPRs we do that
correctly, but for the sink DPRX we only do this correctly
for some platforms.

On pre-HSW (where we don't implement the .set_idle_link_train()
hook), we directly switch from transmitting the training pattern
to normal pixel transmission (the hardware should hopefully
guarantee that the minimum number of required idle patters will
be transmitted during this transition). The DPCD write correctly
precedes the actual switch away from the training pattern.

For HSW+ we start transmitting the idle pattern earlier, and only
switch off the DPCD training pattern after we switch from the idle
pattern to normal pixel transmission. Adjust the code to disable
the DPCD training pattern before we start transmitting the idle
pattern.

v2: Tweak the commit message a bit

Tested-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250710201718.25310-4-ville.syrjala@linux.intel.com

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