gallium: Always map for READ flag when DISCARD is not set.

This prevents the driver from discarding a buffer when the whole buffer
is mapped for writing, but only a portion is effectively written.

This is a temporary fix, because WRITE shouldn't imply DISCARD.

The full fix implies using PIPE_BUFFER_USAGE_DISCARD, throughout
the code, and will go only into master.

src/gallium/include/pipe/p_inlines.h

@@ -63,6 +63,13 @@ pipe_buffer_map(struct pipe_screen *screen,
    if(screen->buffer_map_range) {
       unsigned offset = 0;
       unsigned length = buf->size;
+
+      /* XXX: Actually we should be using/detecting DISCARD
+       * instead of assuming that WRITE implies discard */
+      if((usage & PIPE_BUFFER_USAGE_CPU_WRITE) &&
+         !(usage & PIPE_BUFFER_USAGE_DISCARD))
+         usage |= PIPE_BUFFER_USAGE_CPU_READ;
+
       return screen->buffer_map_range(screen, buf, offset, length, usage);
    }
    else