panfrost: Don't double-free when handling error for unsupported GPU

Setting the screen ro member before we checked gpu id means the error
case leads to a double-free because screen->ro is set and allocated
by parent who hanbdles de-alloc a second time after we destroyed the
screen we just created because ro was set.

Cc: mesa-stable

Signed-off-by: Carsten Haitzler <carsten.haitzler@foss.arm.com>
Part-of: <!14516>
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