freedreno/a6xx: Add registers for the bindless model

freedreno/a6xx: Add registers for the bindless model

In Vulkan, descriptors for samplers, SSBO's, etc. are collected into
descriptor sets, and shaders can use multiple descriptor sets. At
command-recording time, users can swap out only some of the descriptor
sets, and the driver is supposed to do the minimum amount necessary to
update any internal binding tables, knowing that only some of the
descriptors have changed.

With the old binding model, focused on GL, where there are separate
tables for each type of resource, we can do somewhat better than now by
preserving descriptors from lower descriptor sets when switching higher
descriptor sets. However we still have to copy around descriptors before
each draw.

At least for a6xx, qualcomm went further, essentially copying the Vulkan
binding model as an alternate way to load resources. There's an array of
registers (actually an array for compute and one for everything else),
where each register holds a pointer to a descriptor set that can contain
various different descriptor types. The descriptors are padded out to 16
dwords, so that every instruction can use an index instead of a dword
offset. It's called "bindless", I think, because it can also be used to
implement the old GL bindless extensions (presumably it allows more
samplers and textures than the old model).

This commit adds the register and cmdstream parts. Next up will be the
instruction encoding.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4358>