drm/fb-helper: Only consider active CRTCs for vblank sync

In the Linux kernel, the following vulnerability has been resolved:

drm/fb-helper: Only consider active CRTCs for vblank sync

Only synchronize fbdev output to the vblank of an active CRTC. Go over
the list of CRTCs and pick the first that matches. Fixes warnings as
the one shown below

[ 77.201354] WARNING: drivers/gpu/drm/drm_vblank.c:1320 at drm_crtc_wait_one_vblank+0x194/0x1cc [drm], CPU#1: kworker/1:7/1867
[ 77.201354] omapdrm omapdrm.0: [drm] vblank wait timed out on crtc 0

This currently happens if the fbdev output is not on CRTC 0.

Atomic and non-atomic drivers require distinct code paths. As for other
fbdev operations, implement both and select the correct one at runtime.

Not finding an active CRTC is not a bug. Do not wait in this case, but
flush the display update as before.

v4:
- avoid possible deadlocks with locking context (Sashiko)
v3:
- drop excessive state validation (Jani)
- acquire plane and CRTC mutices (Sashiko)
v2:
- move look-up code into separate helper
- support drivers with legacy modesetting
v1:
- see https://lore.kernel.org/dri-devel/1c9e0e24-9c4a-4259-8700-cf9e5fd60ca3@suse.de/