drm/komeda: Fix handling of atomic commits in the atomic_commit_tail hook

drm/komeda: Fix handling of atomic commits in the atomic_commit_tail hook

Komeda driver relies on the generic DRM atomic helper functions to handle
commits. It only implements an atomic_commit_tail hook for the
mode_config_helper_funcs and even that one is pretty close to the generic
implementation with the exception of additional dma_fence signalling.

What the generic helper framework doesn't do is waiting for the actual
hardware to signal that the commit parameters have been written into the
appropriate registers. As we signal CRTC events only on the irq handlers,
we need to flush the configuration and wait for the hardware to respond.

Add the Komeda specific implementation for atomic_commit_hw_done() that
flushes and waits for flip done before calling drm_atomic_helper_commit_hw_done().

The fix was prompted by a patch from Carsten Haitzler where he was trying to
solve the same issue but in a different way that I think can lead to wrong
event signaling to userspace.

Reported-by: Carsten Haitzler <carsten.haitzler@arm.com>
Tested-by: Carsten Haitzler <carsten.haitzler@arm.com>
Reviewed-by: Carsten Haitzler <carsten.haitzler@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220722122139.288486-1-liviu.dudau@arm.com