radeonsi: use DMA for clears with unaligned size

radeonsi: use DMA for clears with unaligned size

Only a small tail needs to be uploaded manually.

This is only partly a performance measure (apps are expected to use
aligned access). Mostly it is preparation for sparse buffers, which the
old code would incorrectly have attempted to map directly.

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