vt(4): Refuse to load a font if hw.vga.textmode is selected

Before, the font was loaded and the window size recalculated, giving an
unusable terminal, even if the actual font didn't change.

Reported by:	beeessdee@ruggedinbox.com
MFC of:		r273330
Merge remote-tracking branch 'freebsd/stable/10' into hardened/10/aslr