drm/lima: add error sysfs to export error task dump

Export /sys/class/drm/cardX/device/error sysfs for user read out
error task dump file.

Tested-by: Andreas Baierl <ichgeh@imkreisrum.de>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200222024210.18697-5-yuq825@gmail.com