linuxkpi: Move pci_request_region and _lkpi_pci_iomap into .c

Both pci_request_region and _lkpi_pci_iomap encode the size of struct
pci_mmio_region into their code. Move from .h to .c files to move that
knowledge from the client drivers into the linuxkpi module.

Sponsored by: Netflix
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34773

(cherry picked from commit 1cdb25340f8ee5dd145b0dc370cbab1bd7bdca65)