i965: Mark needed downsamples for msaa winsys buffers

i965: Mark needed downsamples for msaa winsys buffers

Add function intel_renderbuffer_set_needs_downsample. It is a no-op
except on multisample winsys buffers shared with DRI2.

Mark the needed downsamples with the new function at two locations:
    - Immediately after drawing is complete.
    - After blitting.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>