vt efifb: add suspend/resume calls

Using the latest NVIDIA driver, upon resuming from suspend with X
running the display remained blank.  Additionally OpenGL applications
that were running triggered a number of error messages from the NVIDIA
driver.

This occurred because the vt efifb back-end did not signal the X server
to release the display before suspending (or to re-acquire it after
resuming).  The NVIDIA driver includes code for smoothly shutting down
and re-initializing the GPU, which was not getting called.

Since the NVIDIA driver doesn't currently support framebuffer devices
and vt is forced to fall back to the efifb back-end, add vd_suspend and
vd_resume members to connect the suspend/resume path.  This ensures the
X server is properly able to re-initialize the display.

PR:		237050
Submitted by:	Erik Kurzinger <ekurzinger@nvidia.com>
Reviewed by:	markj
MFC after:	2 weeks
Event:		Waterloo Hackathon 2019

sys/dev/vt/hw/efifb/efifb.c

@@ -64,6 +64,8 @@ static struct vt_driver vt_efifb_driver = {
 	.vd_setpixel = vt_fb_setpixel,
 	.vd_fb_ioctl = vt_fb_ioctl,
 	.vd_fb_mmap = vt_fb_mmap,
+	.vd_suspend = vt_suspend,
+	.vd_resume = vt_resume,
 	/* Better than VGA, but still generic driver. */
 	.vd_priority = VD_PRIORITY_GENERIC + 1,
 };