venus: pm_helpers: Fix error check in vcodec_domains_get()

In the function vcodec_domains_get(), dev_pm_domain_attach_by_name()
may return NULL in some cases, so IS_ERR() doesn't meet the
requirements. Thus fix it.

Fixes: 7482a983 ("media: venus: redesign clocks and pm domains control")
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>