i915: Disable the depth test whenever we don't have a depth buffer.

i915: Disable the depth test whenever we don't have a depth buffer.

We were disabling it once at the moment we changed draw buffers, but
later enabling of depth test could turn it back on.  Fixes
fbo-nodepth-test.

Note that ctx->DrawBuffer has to be checked because during context
create we get called while it's still unset.  However, we know we'll
get an intel_draw_buffer() after that, so it's safe to make a silly
choice at this point.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30080