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