drm/i915/dp: Rename intel_dp::link_trained to link.active

drm/i915/dp: Rename intel_dp::link_trained to link.active

The intel_dp::link_trained flag indicates whether the link is active,
regardless of whether the link training passed or failed. For clarity
rename the flag to 'active'. While at it move the flag under
intel_dp::link.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250404150310.1156696-2-imre.deak@intel.com