drm/vmwgfx: clean up some error pointer checking

The vmw_user_bo_noref_lookup() function cannot return NULL.  If it
could, then this function would return PTR_ERR(NULL) which is success.
Returning success without initializing "*vmw_bo_p = vmw_bo;" would
lead to an uninitialized variable bug in the caller.  Smatch complains
about this:

drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c:1177 vmw_translate_mob_ptr() warn: passing zero to 'PTR_ERR'
drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c:1314 vmw_cmd_dx_bind_query() error: uninitialized symbol 'vmw_bo'.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/YtZ9qrKeBqmmK8Hv@kili

drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c

@@ -1172,7 +1172,7 @@ static int vmw_translate_mob_ptr(struct vmw_private *dev_priv,
 
 	vmw_validation_preload_bo(sw_context->ctx);
 	vmw_bo = vmw_user_bo_noref_lookup(sw_context->filp, handle);
-	if (IS_ERR_OR_NULL(vmw_bo)) {
+	if (IS_ERR(vmw_bo)) {
 		VMW_DEBUG_USER("Could not find or use MOB buffer.\n");
 		return PTR_ERR(vmw_bo);
 	}
@@ -1226,7 +1226,7 @@ static int vmw_translate_guest_ptr(struct vmw_private *dev_priv,
 
 	vmw_validation_preload_bo(sw_context->ctx);
 	vmw_bo = vmw_user_bo_noref_lookup(sw_context->filp, handle);
-	if (IS_ERR_OR_NULL(vmw_bo)) {
+	if (IS_ERR(vmw_bo)) {
 		VMW_DEBUG_USER("Could not find or use GMR region.\n");
 		return PTR_ERR(vmw_bo);
 	}