drm/fb-helper: Synchronize dirty worker with vblank

Before updating the display from the console's shadow buffer, the dirty
worker now waits for a vblank. This allows several screen updates to pile
up and acts as a rate limiter. If a DRM master is present, it could
interfere with the vblank. Don't wait in this case.

v4:
	* share code with WAITFORVSYNC ioctl (Emil)
	* use lock guard
v3:
	* add back helper->lock
	* acquire DRM master status while waiting for vblank
v2:
	* don't hold helper->lock while waiting for vblank

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20250829091447.46719-1-tzimmermann@suse.de

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