drm/atomic: fix out of bounds read in for_each_*_in_state helpers

for_each_*_in_state validate array index after
access to array elements, thus perform out of bounds read.

Fix this by validating index in the first place and read
array element iff validation was successful.

Fixes: df63b999

 ("drm/atomic: Add for_each_{connector,crtc,plane}_in_state helper macros")
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>