mesa: Disallow GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME on winsys FBO.

mesa: Disallow GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME on winsys FBO.

Fixes:
dEQP-GLES3.functional.negative_api.state.get_framebuffer_attachment_parameteriv

Apparently, GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME is not allowed when
GL_FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE is GL_FRAMEBUFFER_DEFAULT, and
is expected to result in a GL_INVALID_ENUM error.

No GL specification actually defines what GL_FRAMEBUFFER_DEFAULT means.
It probably means the window system FBO.  It also doesn't mention the
behavior of any queries for that type.  Various ARB folks seem fairly
confused about it too.  For now, just do something vaguely like what
dEQP expects.

I think we probably need to check the visual bits against 0 for the
attachment, but we haven't been doing that thusfar, and given how
confusingly this is specified, I can't imagine anyone relying on it.

v2: Improve comments, move error condition above the
    _mesa_get_fb0_attachment call, add forgotten "return"
    (all suggested/caught by Jordan Justen).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>