drm/i915/gvt: Fix the vblank timer close issue after shutdown VMs in reverse

Once the Windows guest is shutdown, the display pipe will be disabled
and intel_gvt_check_vblank_emulation will be called to check if the
vblank timer is turned off. Given the scenario of creating VM1 ,VM2,
destoying VM2 in current code, VM1 has pipe enabled and continues to
check VM2, the flag have_enabled_pipe is always false since all the VM2
pipes are disabled, so the vblank timer will be canceled and TDR happens
in Windows VM1 guest due to the vsync timeout.

In this patch the vblank timer will be never canceled once one pipe is
enabled.

v2:
- remove have_enabled_pipe flag and check pipe enabled directly. (Zhenyu)

Cc: Wang Hongbo <hongbo.wang@intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>