panfrost: Set rt_count

panfrost: Set rt_count

This doesn't quite work yet, but it illustrates how MRT is implemented
in the MFBD: rt_count is set appropriately based on the number of render
targets, while additional render target descriptors are appended on with
an index variable in them (not quite decoded since there's some aspects
we don't understand there, but conceptually this should be right).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>