intel/dev: Bail when INTEL_DEVID_OVERRIDE is not valid

intel/dev: Bail when INTEL_DEVID_OVERRIDE is not valid

Avoids surprises where you set an OVERRIDE but it gets ignored and the
system PCI ID is used.

Also fixes the bug that the error of invalid platform name being
printed too early, even when the passed platform was a PCI ID (which
is also supported).

For the case where euid != uid, a warning was added but the behavior
wasn't changed: it is still going to fallback to system PCI ID.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4841>