nv50,nvc0: allow to create resources other than buffers

nv50,nvc0: allow to create resources other than buffers

For the compute support, we might stick buffers as surfaces. This fixes
an assertion when executing src/gallium/tests/trivial/compute.

To avoid using these "restricted" surfaces as render targets, these
assertions have been moved. Note that it's already handled for the
framebuffer thing on nvc0.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>