Use MTX_SYSINIT for the VESA lock.

vesa_init_done isn't a reliable guard for the mutex init. If
vesa_configure() doesn't find valid VESA info it will not set
vesa_init_done, but the lock will remain initialized. Revert r303076
and use MTX_SYSINIT to deterministically init the lock.

Reviewed by: royger
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D7290