xf86drm: fallback to normal path when realpath fails

Earlier commit reworked our sysfs handling to use realpath.
Sadly that backfired since the Firefox sandboxing mechanism rejects
that. Despite the files/folders being in the allowed list, of the
sandboxing mechanism.

Oddly enough, the Chromium sandboxing doesn't complain about any of
this.

Since there are no Firefox releases with the fix, add a temporary
solution which falls back to the original handling.

Sadly, this won't work for virgl.

v2: drop return type - function cannot return NULL (Eric)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107516


Fixes: a0290013 ("xf86drm: introduce a get_real_pci_path() helper")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>