Disabling the display using MCDE currently results in a warning
together with a delay caused by some timeouts:

    mcde a0350000.mcde: MCDE display is disabled
    ------------[ cut here ]------------
    WARNING: CPU: 0 PID: 20 at drivers/gpu/drm/drm_atomic_helper.c:2258 drm_atomic_helper_commit_hw_done+0xe0/0xe4
    Hardware name: ST-Ericsson Ux5x0 platform (Device Tree Support)
    Workqueue: events drm_mode_rmfb_work_fn
    [<c010f468>] (unwind_backtrace) from [<c010b54c>] (show_stack+0x10/0x14)
    [<c010b54c>] (show_stack) from [<c079dd90>] (dump_stack+0x84/0x98)
    [<c079dd90>] (dump_stack) from [<c011d1b0>] (__warn+0xb8/0xd4)
    [<c011d1b0>] (__warn) from [<c011d230>] (warn_slowpath_fmt+0x64/0xc4)
    [<c011d230>] (warn_slowpath_fmt) from [<c0413048>] (drm_atomic_helper_commit_hw_done+0xe0/0xe4)
    [<c0413048>] (drm_atomic_helper_commit_hw_done) from [<c04159cc>] (drm_atomic_helper_commit_tail_rpm+0x44/0x6c)
    [<c04159cc>] (drm_atomic_helper_commit_tail_rpm) from [<c0415f5c>] (commit_tail+0x50/0x10c)
    [<c0415f5c>] (commit_tail) from [<c04160dc>] (drm_atomic_helper_commit+0xbc/0x128)
    [<c04160dc>] (drm_atomic_helper_commit) from [<c0430790>] (drm_framebuffer_remove+0x390/0x428)
    [<c0430790>] (drm_framebuffer_remove) from [<c0430860>] (drm_mode_rmfb_work_fn+0x38/0x48)
    [<c0430860>] (drm_mode_rmfb_work_fn) from [<c01368a8>] (process_one_work+0x1f0/0x43c)
    [<c01368a8>] (process_one_work) from [<c0136d48>] (worker_thread+0x254/0x55c)
    [<c0136d48>] (worker_thread) from [<c013c014>] (kthread+0x124/0x150)
    [<c013c014>] (kthread) from [<c01010e8>] (ret_from_fork+0x14/0x2c)
    Exception stack(0xeb14dfb0 to 0xeb14dff8)
    dfa0:                                     00000000 00000000 00000000 00000000
    dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    dfe0: 00000000 00000000 00000000 00000000 00000013 00000000
    ---[ end trace 314909bcd4c7d50c ]---
    [drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CRTC:32:crtc-0] flip_done timed out
    [drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [CONNECTOR:34:DSI-1] flip_done timed out
    [drm:drm_atomic_helper_wait_for_dependencies] *ERROR* [PLANE:31:plane-0] flip_done timed out

The reason for this is that there is a vblank event pending, but we
never handle it after disabling the vblank interrupts.

Check if there is an vblank event pending when disabling the display,
and clear it by sending a fake vblank event in that case.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106165835.2863-8-stephan@gerhold.net