drm/atomic-helpers: Stall on the right commit

stall_checks carefully picked out the right commit to stall on, then
promptly used the wrong variable. Due to the break in the next loop
iteration this could be the 3rd commit, or if the list only has 2
entries commit would now point into the struct drm_crtc itself, at
some offset. Hilarity eventually ensues.

For added safety, also break right away instead of iterating once
more, but the real fix is waiting on stall_commit instead of commit.

Reported-and-tested-by: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465926658-10110-1-git-send-email-daniel.vetter@ffwll.ch