drm/xen-front: Fix misused IS_ERR_OR_NULL checks

drm/xen-front: Fix misused IS_ERR_OR_NULL checks

The patch c575b7eeb89f: "drm/xen-front: Add support for Xen PV
display frontend" from Apr 3, 2018, leads to the following static
checker warning:

	drivers/gpu/drm/xen/xen_drm_front_gem.c:140 xen_drm_front_gem_create()
	warn: passing zero to 'ERR_CAST'

drivers/gpu/drm/xen/xen_drm_front_gem.c
   133  struct drm_gem_object *xen_drm_front_gem_create(struct drm_device *dev,
   134                                                  size_t size)
   135  {
   136          struct xen_gem_object *xen_obj;
   137
   138          xen_obj = gem_create(dev, size);
   139          if (IS_ERR_OR_NULL(xen_obj))
   140                  return ERR_CAST(xen_obj);

Fix this and the rest of misused places with IS_ERR_OR_NULL in the
driver.

Fixes:  c575b7eeb89f: "drm/xen-front: Add support for Xen PV display frontend"

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200813062113.11030-3-andr2000@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>