Ignore kern.vt.splash_cpu without graphics

When the system has no graphical console, such as bhyve in common
configurations, ignore kern.vt.splash_cpu, instead of panicking
on INVARIANTS kernels.

Reviewed by: cem dumbbell
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D20877