drm/atomic: Make atomic iterators less surprising

Commit 669c9215 ("drm/atomic: Make async plane update checks work as
intended, v2.") assumed incorrectly that if only 1 plane is matched in
the loop, the variables will be set to that plane. In reality we reset
them to NULL every time a new plane was iterated. This behavior is
surprising, so fix this by making the for loops only assign the
variables on a match.

When we have not added all the planes/crtc/connector to the state, and
there's a few NULL ones after the last one we iterated, te assumption
is broken that the pointers will hold the values from the last loop
iteration, which holds true for all other for_each macros we're using.
Except of course the iterator pointer itself, but that one really is
entirely internal.

Cc: Dmitry Osipenko <digetx@gmail.com>
Fixes: 669c9215 ("drm/atomic: Make async plane update checks work as intended, v2.")
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927083532.5756-2-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Dmitry Osipenko <digetx@gmail.com>