mesa: Fix the cause of piglit test fbo-array failure

mesa: Fix the cause of piglit test fbo-array failure

Handle the special case of glFramebufferTextureLayer() for which we pass
teximage = 0 internally in framebuffer_texture(). This patch makes failing
piglit test fbo-array, fbo-depth-array to pass.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47126

V4: Removed the duplicated code.
Note: This is a candidate for the stable branches.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>