intel: Fix segfaults from trying to use _ColorDrawBuffers in FBO validation.

intel: Fix segfaults from trying to use _ColorDrawBuffers in FBO validation.

The _ColorDrawBuffers is a piece of computed state that gets for the
current draw/read buffers at _mesa_update_state time.  However, this
function actually gets used for non-current draw/read buffers when
checking if an FBO is complete from the driver's perspective.  So,
instead of trying to just look at the attachment points that are
currently referenced by glDrawBuffers, look at all attachment points
to see if they're driver-supported formats.  This appears to actually
be more in line with the intent of the spec, too.

Fixes a segfault in my upcoming fbo-clear-formats piglit test, and
hopefully bug #30278