drm/radeon: Make classic pageflip completion path less racy.

drm/radeon: Make classic pageflip completion path less racy.

Need to protect mmio flip programming by event lock as well.

Need to also first enable pflip irq, then mmio program,
otherwise a flip completion may get unnoticed in the vblank
of actual completion if the flip is programmed, but
radeon_flip_work_func gets preempted immediately after
mmio programming and before vblank. In that case the
vblank irq handler wouldn't run radeon_crtc_handle_vblank()
with the completion check routine, miss the completed flip,
and only notice one vblank after actual completion, causing
a false/delayed report of flip completion.

Signed-off-by: Mario Kleiner <[email protected]>
Reviewed-by: Michel Dänzer <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>