panfrost: Make the sysval logic more generic

We are about to add support for nir_texop_txs which requires adding a
sysval/uniform containing the texture size. Let's change the
emit_sysval_read() prototype to take a nir_instr object instead of
a nir_intrinsic_instr one so we can re-use this function when emitting
a sysval for a txs instruction.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>