st/mesa: remove unneeded pipe_surface_release() in st_render_texture()

st/mesa: remove unneeded pipe_surface_release() in st_render_texture()

This caused us to always free the pipe_surface for the renderbuffer.
The subsequent call to st_update_renderbuffer_surface() would typically
just recreate it.  Remove the call to pipe_surface_release() and let
st_update_renderbuffer_surface() take care of freeing the old surface
if it needs to be replaced (because of change to mipmap level, etc).

This can save quite a few calls to pipe_context::create_surface() and
surface_destroy().

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>