vt_core: make sure the driver's functions exist

[vexeduxr]

Summary

These are NULL if they're not implemented. Make sure all the functions
we need are there before doing anything.

Also invert the first if statment to lessen the indentation a bit.

Reported by: Quentin Thébault <[e-mail]>
MFC after: 3 days

[vexeduxr]

Bump.
I'd like to get this into stable/15 before releng/15.1 branches

[kevans]

sys/dev/vt/vt_core.c

1687–1701

I was pondering whether we really want to set VDF_SPLASH if some of this can't happen due to missing callbacks, and whether we really want to try and draw a shutdown splash if we can't blank the screen. I tend to think that bailing out entirely if vd_blank is missing is probably the right answer for both questions?

[vexeduxr]

sys/dev/vt/vt_core.c

1687–1701

Hmm, yes. Maybe we also shouldn't blank the screen if vd_bitblt_* are missing? (depending on which one we want to call).
I'll try restructuring the function a bit.

[vexeduxr]

Don't do anything until we know that all the functions we need are present.
Also invert the first if statment to lessen the indentation.

[vexeduxr]

Small style tweak