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