linuxkpi: hdmi: Split the module declaration to a new file

In order to have a proper linuxkpi_video kmod, move the module declaration
to a new file as linuxkpi_video will also include linux_hdmi.c

Differential Revision: https://reviews.freebsd.org/D44926
Reviewed by: bz, emaste, wulf
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 7f84bb34a1d191949ec519a01c0c574f5e827c88)