[bug report] drm/xen-front: Add support for Xen PV display frontend

[Dan Carpenter]

It turns out there aren't that many of these in xen.

$ grep IS_ERR_OR_NULL drivers/gpu/drm/xen/ -Rn
drivers/gpu/drm/xen/xen_drm_front_kms.c:63:     if (IS_ERR_OR_NULL(fb))
drivers/gpu/drm/xen/xen_drm_front_gem.c:86:     if (IS_ERR_OR_NULL(xen_obj))
drivers/gpu/drm/xen/xen_drm_front_gem.c:120:    if (IS_ERR_OR_NULL(xen_obj->pages)) {
drivers/gpu/drm/xen/xen_drm_front_gem.c:139:    if (IS_ERR_OR_NULL(xen_obj))
drivers/gpu/drm/xen/xen_drm_front_gem.c:197:    if (IS_ERR_OR_NULL(xen_obj))
drivers/gpu/drm/xen/xen_drm_front.c:403:        if (IS_ERR_OR_NULL(obj)) {

They're all wrong, because if the pointer was really NULL then it's
not handled correctly and would eventually lead to a runtime failure.

Normally Smatch is smart enough to know that the pointer isn't NULL so
it doesn't generate a warning but yesterday I introduced a bug in Smatch
by mistake.  It's fixed now.

regards,
dan carpenter