mesa: check fbo completeness based on internal format, not driver format

mesa: check fbo completeness based on internal format, not driver format

The base format is a function of the user-requested format, while the
driver format is not. So we should use the base format instead.

The driver format can be anything. Specifically in the stencil-only
case, it might be a depth/stencil format. However we still want to
refuse such an attachment when bound to GL_DEPTH.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Brian Paul <brianp@vmware.com>