drm/msm: Use for_each_*_in_state

We want to hide drm_atomic_state internals

v2: Review from Maarten:
- remove whitespace change in rockchip driver that slipped in.
- use drm_crtc_mask insted of open-coding it.

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-4-git-send-email-daniel.vetter@ffwll.ch