drm/i915: use drm_modeset_lock_assert_held() in intel_connector_get_pipe()

drm/i915: use drm_modeset_lock_assert_held() in intel_connector_get_pipe()

In the intel_connector_get_pipe() function, we check if
connection_mutex is held and generate our own WARN_ON if that's the
case.  Instead of generating a non-standard warning for a mutex issue,
we should use the standard lockdep framework.

Change the function to use drm_modeset_lock_assert_held() instead.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250520095408.1310440-1-luciano.coelho@intel.com