drm/vmwgfx: Remove drm_crtc_arm_vblank_event from atomic flush

The function drm_crtc_arm_vblank_event should be used for the driver
which have vblank interrupt support. In case of vmwgfx we do not have
vblank interrupt.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>