vc4: Don't leak the GPU fd for renderonly usage.

Noticed while debugging V3D -- the ro->gpu_fd was freshly opened in ro
setup, and it needs to stay open until screen close (since it may be used
by renderonly) and should be the same one used by the vc4 screen.

Fixes: 7029ec05 ("gallium: Add renderonly-based support for pl111+vc4.")
(cherry picked from commit 99ef66c3)