drm/i915/gvt: Add missing breaks in switch statement

The switch statement is missing breaks for the cases of
GVT_FAILSAFE_INSUFFICIENT_RESOURCE and GVT_FAILSAFE_GUEST_ERR. Add them
in.

Detected by CoverityScan, CID#1462416 ("Missing break in switch")

Fixes: e011c6ce ("drm/i915/gvt: Add VM healthy check for workload_thread")
Fixes: a33fc7a0 ("drm/i915/gvt: enter failsafe mode when guest requires more resources")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>