i386: Avoid registering overlapping vm_phys_seg entries

The entry added in pmap_bootstrap() is guaranteed to be covered by that
added in pmap_cold(). This apparently went unnoticed until commit
8a14ddcc1d8e ("vm_phys: Check for overlap when adding a segment").

PR: 285415
Tested by: dim
MFC after: 2 weeks

(cherry picked from commit e173855c9f6b3b87c975418d0acf1c22ac4f1626)