radeon/winsys: pad IBs to a multiple of 8 DWs

radeon/winsys: pad IBs to a multiple of 8 DWs

This aligns the gfx, compute, and dma IBs to 8 DW boundries.
This aligns the the IB to the fetch size of the CP for optimal
performance. Additionally, r6xx hardware requires at least 4
DW alignment to avoid a hw bug.  This also aligns the DMA
IBs to 8 DW which is required for the DMA engine.  This
alignment is already handled in the gallium driver, but that
patch can be removed now that it's done in the winsys.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: "9.2" <mesa-stable@lists.freedesktop.org>
CC: "9.1" <mesa-stable@lists.freedesktop.org>