linuxkpi: Restore the KBI for struct pci_driver

The size of the 13.0 version of struct pci_driver was 92 or 184 bytes on
32- or 64-bit systems respectively. We recently added bsd_probe_return
at the end of this struct, breaking the KBI on the stable/13 branch.

Fix this by removing the isdrm member. We don't need it because we can
do a strcmp in the few places that need it as they aren't performance
critical. Move the newly added bsd_probe_return to that slot. It's the
same size in all our supported KBIs as bool and fits into that slot due
to padding rules.

Direct commit to stable/13 because this is not relevant to main.

Sponsored by: Netflix
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D34754