[media] v4l: vsp1: drm: Include correct header file

The VSP1 DRM API is declared in <media/vsp1.h>, not <linux/vsp1.h>. Fix
it. This also reverts commit 18922936

 ("[media] vsp1_drm.h: add
missing prototypes") that added the same declarations in a different
header file.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>