drm/atmel: Use drm_atomic_helper_commit

One of these drivers that predates the nonblocking support in helpers,
and hand-rolled its own thing. Entirely not anything specific here, we
can just delete it all and replace it with the helper version.

Could also perhaps use the drm_mode_config_helper_suspend/resume
stuff, for another few lines deleted. But I'm not looking at that
stuff, I'm just going through all the atomic commit functions and make
sure they have properly annotated dma-fence critical sections
everywhere.

v2:
- Also delete the workqueue (Sam)
- drop the @commit kerneldoc, I missed that one.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Boris Brezillon <bbrezillon@kernel.org>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210121152959.1725404-7-daniel.vetter@ffwll.ch