glx: Generate fewer errors in MakeContextCurrent

glx: Generate fewer errors in MakeContextCurrent

For a few reasons.

1: In the (current) common case, these conditionals are never true. All
we're doing by checking them is slowing down MakeCurrent.  The server
does these checks already anyway.

2: GLX >= 3.0 contexts may legally be made current without a bound
framebuffer.

This does not fix piglit/glx-create-context-current-no-framebuffer, but
is a prerequisite for fixing it.

Cc: "9.1 9.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>