media: qcom: venus: fix incorrect return value

'pd' can be NULL, and in that case it shouldn't be passed to
PTR_ERR. Fixes a smatch warning:

drivers/media/platform/qcom/venus/pm_helpers.c:873 vcodec_domains_get() warn: passing zero to 'PTR_ERR'

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>