drm/vkms: Bugfix extra vblank frame

drm/vkms: Bugfix extra vblank frame

kms_flip tests are breaking on vkms when simulate vblank because vblank
event sequence count returns one extra frame after arm vblank event to
make a page flip.

When vblank interrupt happens, userspace processes the vblank event and
issues the next page flip command. Kernel calls queue_work to call
commit_planes and arm the new page flip. The next vblank picks up the
newly armed vblank event and vblank interrupt happens again.

The arm and vblank event are asynchronous, then, on the next vblank, we
receive x+2 from `get_vblank_timestamp`, instead x+1, although timestamp
and vblank seqno matches.

Function `get_vblank_timestamp` is reached by 2 ways:

  - from `drm_mode_page_flip_ioctl`: driver is doing one atomic
    operation to synchronize planes in the same output. There is no
    vblank simulation, the `drm_crtc_arm_vblank_event` function adds 1
    on vblank count, and the variable in_vblank_irq is false
  - from `vkms_vblank_simulate`: since the driver is doing a vblank
    simulation, the variable in_vblank_irq is true.

Fix this problem subtracting one vblank period from vblank_time when
`get_vblank_timestamp` is called from trace `drm_mode_page_flip_ioctl`,
i.e., is not a real vblank interrupt, and getting the timestamp and
vblank seqno when it is a real vblank interrupt.

The reason for all this is that get_vblank_timestamp always supplies the
timestamp for the next vblank event. The hrtimer is the vblank
simulator, and it needs the correct previous value to present the next
vblank. Since this is how hw timestamp registers work and what the
vblank core expects.

Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/171e6e1c239cbca0c3df7183ed8acdfeeace9cf4.1548856186.git.shayenneluzmoura@gmail.com