winsys/amdgpu: start with smaller IBs, growing as necessary

winsys/amdgpu: start with smaller IBs, growing as necessary

This avoids allocating giant IBs from the outset, especially for CE and DMA.

Since we now limit max_dw only by the size that the buffer happens to be
(which, due to the buffer cache, can be even larger than the rounded-up size
we request), the new function amdgpu_ib_max_submit_dwords controls when we
submit an IB.

With this change, we effectively never flush prematurely due to the CE IB,
after an initial warm-up phase.

v2:
- clean up buffer_size calculation

Reviewed-by: Marek Olšák <marek.olsak@amd.com>