backend/session: use DRM_PRIMARY_MINOR_NAME
backend/session: use DRM_PRIMARY_MINOR_NAME

Instead of hardcoding the string "card", use DRM_PRIMARY_MINOR_NAME.
Some systems may use another prefix, e.g. OpenBSD uses "drm" instead.

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots