drm/vkms: guarantee vblank when capturing crc

VKMS needs vblank interrupts enabled to capture CRC. When vblank is
disabled, tests like kms_cursor_crc and kms_pipe_crc_basic getting stuck
waiting for a capture that will not occur until vkms wakes up. This patch
adds a helper to set composer and ensure that vblank remains enabled as
long as the CRC capture is needed.

It clears the execution of the following kms_cursor_crc subtests:
1. pipe-A-cursor-[size,alpha-opaque, NxN-(on-screen, off-screen, sliding,
random, fast-moving])] - successful when running individually.
2. pipe-A-cursor-dpms passes again
3. pipe-A-cursor-suspend also passes

The issue was initially tracked in the sequential execution of IGT
kms_cursor_crc subtests: when running the test sequence or one of its
subtests twice, the odd execs complete and the pairs get stuck in an
endless wait. In the IGT code, calling a wait_for_vblank on preparing for
CRC capture prevented the busy-wait. But the problem persisted in the
pipe-A-cursor-dpms and -suspend subtests.

Checking the history, the pipe-A-cursor-dpms subtest was successful when,
in vkms_atomic_commit_tail, instead of using the flip_done op, it used
wait_for_vblanks. Another way to prevent blocking was wait_one_vblank when
enabling crtc. However, in both cases, pipe-A-cursor-suspend persisted
blocking in the 2nd start of CRC capture, which may indicate that
something got stuck in the step of CRC setup. Indeed, wait_one_vblank in
the crc setup was able to sync things and free all kms_cursor_crc
subtests. Besides, other alternatives to force enabling vblanks or prevent
disabling them such as calling drm_crtc_put_vblank or modeset_enables
before commit_planes + offdelay = 0, also unlock all subtests executions.

Finally, due to vkms's dependence on vblank interruptions to perform
tasks, this patch uses refcount to ensure that vblanks happen when
enabling composer and while crc capture is needed.

Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>

v2:
- extract a vkms_set_composer helper
- fix vblank refcounting for the disabling case

v3:
- make the vkms_set_composer helper static
- review the credit tags

Co-debugged-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: add changelog back in]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200808120900.pudwwrfz44g3rqx7@smtp.gmail.com