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>