drm/atomic: Introduce state->obj backpointers

Useful since this way we can pass around just the state objects and
will get ther real object, too.

Specifically this allows us to again simplify the parameters for
set_crtc_for_plane.

v2: msm already has it's own specific plane_reset hook, don't forget
that one!

v3: Fixup kerneldoc, reported by 0-day builder.

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com> (v2)
Tested-by: Rob Clark <robdclark@gmail.com> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>