softpipe: don't assert when creating surfaces with multiple layers

softpipe: don't assert when creating surfaces with multiple layers

We can't handle them yet, however we can safely just warn (we will
just render to first layer, which is fine since we can't handle
rendertarget system value neither).
Also make behavior more predictable with buffer surfaces
(it would sometimes hit bogus asserts because of the union in the surface,
instead create the surface but assert when trying to set a buffer
in the framebuffer).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>