drm/drv: Remove invalid assignments

it's not necessary to assign a value of 0 to ret here, because if
the previous functions were executed correctly, ret would be 0.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/1607653037-37785-1-git-send-email-tiantao6@hisilicon.com