drm/msm: update cursors asynchronously through atomic

Add support to async updates of cursors by using the new atomic
interface for that. Basically what this commit does is do what
mdp5_update_cursor_plane_legacy() did but through atomic.

v5: call drm_atomic_helper_async_check() from the check hook

v4: add missing atomic async commit call to msm_atomic_commit(Archit Taneja)

v3: move size checks back to drivers (Ville Syrjälä)

v2: move fb setting to core and use new state (Eric Anholt)

Cc: Rob Clark <robdclark@gmail.com>
Cc: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Tested-by: Archit Taneja <architt@codeaurora.org> (v4)
[added comment about not hitting async update path if hwpipes are
 re-assigned or global state is touched]
Signed-off-by: Rob Clark <robdclark@gmail.com>