st/mesa: fix changing internal format via RenderbufferStorage

st/mesa: fix changing internal format via RenderbufferStorage

The problem is: The second time the function is called with a new
internal format, strb->format is usually not PIPE_FORMAT_NONE.

RenderbufferStorage(... GL_RGBA8 ...);
RenderbufferStorage(... GL_RGBA16 ...); // had no effect on the format

Broken with: fd6f2d6e5783d8810d0ab88e1c470958fd5eb2eb
Test: piglit/fbo-storage-completeness

NOTE: This is a candidate for the 7.10 branch.
(if fd6f2d6e5783d8810d0ab88e1c470958fd5eb2eb is cherry-picked as well)

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