drm/i915/gvt: Fix drm_WARN issue where vgpu ptr is unavailable

When vgpu ptr is unavailable, the drm_WARN* can hang the whole system
due to the drm pointer is NULL. This patch fixes this issue by using
WARN directly which won't care about the drm pointer.

Fixes: 12d58619 ("drm/i915/gvt: Make WARN* drm specific where vgpu ptr is available")
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200225053527.8336-1-tina.zhang@intel.com