media: venus: core: Fix a potential NULL pointer dereference in an error handling path

The normal path of the function makes the assumption that
'pm_ops->core_power' may be NULL.
We should make the same assumption in the error handling path or a NULL
pointer dereference may occur.

Add the missing test before calling 'pm_ops->core_power'

Fixes: 9e8efdb5 ("media: venus: core: vote for video-mem path")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>