drm/i915/lspcon: remove dp_to_lspcon(), hide enc_to_intel_lspcon()

There are no users left for dp_to_lspcon(), remove it. The only two
users of enc_to_intel_lspcon() are in intel_lspcon.c, so hide it there.

v2: Rebase

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ea14ba07b88ba5ed65d832c89a3415b7a91edea9.1738677489.git.jani.nikula@intel.com


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