panfrost: Fix ->reads_frag_coord assignment

Let's assign ->reads_frag_coord only once to handle the sysval case
(used on Bifrost) correctly.

Fixes: f1de952b ("panfrost: Use shader_info harder")
Cc: 20.3 <mesa-stable>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!7527>