drm/vkms: Bugfix racing hrtimer vblank handle

When the vblank irq happens, kernel time subsystem executes
`vkms_vblank_simulate`. In parallel or not, it prepares all stuff
necessary to the next vblank with arm, and it must flush these stuff
before the next vblank irq. However, vblank counter is ahead when arm is
executed in parallel with handle vblank.

CPU 0:					CPU 1:
 |					 |
atomic_commit_tail is ongoing		 |
 |					 |
 |					hrtimer: vkms_vblank_simulate()
 |					 |
 |					drm_crtc_handle_vblank()
 |					 |
drm_crtc_arm_vblank()			 |
 |					 |
->get_vblank_timestamp()		 |
 |					 |
 |					hrtimer_forward_now()

Then, we should guarantee that the vblank interval time is correct (not
changed) before finish the vblank handle.

Fix the bug including the call to `hrtimer_forward_now()` in the same
lock of `drm_crtc_handle_vblank()` to ensure that the timestamp update
is correct when finish the vblank handle.

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/e2e4b8f3a5cab7b2dba75bf1930f86b0a4ee08c9.1548856186.git.shayenneluzmoura@gmail.com