Since 8059add04 "drm: allow render capable master with DRM_AUTH ioctls" in
the kernel, the assumption that the fd from a primary node that is working
for rendering is a master fd, is not true anymore.

This causes issues in that if we already have a "master fd", leases stop
working as it only allows 1 master fd for the wsi.

This open codes drmIsMaster because I don't think we should bump libdrm
requirements in backports.

Fixes: 451b58a5 "radv: Add KHR_display extension to radv [v5]"