mesa/st: Make sure we do the resolve blit for MSAA winsys readpixels.
Usually it would work out because the user would ask for RGBA8 on a BGRA8 RB, causing the staging blit to do the resolve. But if you had an RGBA8 winsys RB (hi, zink!), then we would skip it and try to pipe_transfer_map() an MSAA resource.
Fixes dEQP tests like dEQP-GLES3.functional.multisample.default_framebuffer.depth on zink when run with a multisample fbconfig.