gallium/radeon: add RADEON_FLAG_HANDLE

gallium/radeon: add RADEON_FLAG_HANDLE

When passed to winsys->buffer_create, this flag will indicate that we require
a buffer that maps 1:1 with a kernel buffer handle.

This is currently set for all textures, since textures can potentially be
exported to other processes. This is not a huge loss, since the main purpose
of this patch series is to deal with applications that allocate many small
buffers.

A hypothetical application with tons of tiny textures might still benefit
from not setting this flag, but that's not a use case I'm worried about
just now.

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