gallium: expose ARB_map_buffer_alignment on Radeon

Reviewed-by: Brian Paul <brianp@vmware.com>

v2: update relnotes-9.1
v3: use align_malloc and align_free for malloced buffers in r300g
v4: document the new CAP in the docs

src/gallium/drivers/softpipe/sp_screen.c

@@ -166,6 +166,7 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
    case PIPE_CAP_VERTEX_ELEMENT_SRC_OFFSET_4BYTE_ALIGNED_ONLY:
    case PIPE_CAP_START_INSTANCE:
    case PIPE_CAP_TEXTURE_MULTISAMPLE:
+   case PIPE_CAP_MIN_MAP_BUFFER_ALIGNMENT:
       return 0;
    case PIPE_CAP_QUERY_TIMESTAMP:
       return 1;