drm/i915/lspcon: add intel_lspcon_active() and use it

Hide the direct lspcon->active use behind intel_lspcon_active().

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3eabe0cb07a84c63119ea893e149de410b4356b0.1738677489.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>