vesa: fix panic on suspend

Fix the following panic seen when migrating a FreeBSD guest on Xen:

panic: mtx_lock() of destroyed mutex @ /usr/src/sys/dev/fb/vesa.c:541
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe001d2fa4f0
vpanic() at vpanic+0x182/frame 0xfffffe001d2fa570
kassert_panic() at kassert_panic+0x126/frame 0xfffffe001d2fa5e0
mtx_lock_flags() at mtx_lock_flags+0x15b/frame 0xfffffe001d2fa630
vesa_bios_save_restore() at vesa_bios_save_restore+0x78/frame 0xfffffe001d2fa680
vga_suspend() at vga_suspend+0xa3/frame 0xfffffe001d2fa6b0
isavga_suspend() at isavga_suspend+0x1d/frame 0xfffffe001d2fa6d0
bus_generic_suspend_child() at bus_generic_suspend_child+0x44/frame
[...]

This is caused because vga_sub_configure (which is called if the VGA adapter
is attached after VESA tried to initialize), points to vesa_configure, which
doesn't initialize the VESA mutex. In order to fix it, make sure
vga_sub_configure points to vesa_load, so that all the needed vesa
components are properly initialized.

Sponsored by: Citrix Systems R&D
MFC after: 3 days
PR: 209203
Reviewed by: dumbbell
Differential revision: https://reviews.freebsd.org/D7196