vt(4): Fix a LOR which occurs during a call to vt_upgrade()

Reported by:	kib@
Review:		https://reviews.freebsd.org/D785
Reviewed by:	ray@
Approved by:	ray@
MFC after:	2 days
vt(4): Use vt_fb_drawrect() and vt_fb_setpixel() in all vt_fb-derivative

Review:		https://reviews.freebsd.org/D789
Reviewed by:	nwhitehorn
Approved by:	nwhitehorn
MFC after:	2 days