llvmpipe: don't assert on illegal surface creation.

llvmpipe: don't assert on illegal surface creation.

Since c8eb2d0e829d0d2aea6a982620da0d3cfb5982e2 llvmpipe checks if it's
actually legal to create a surface. The opengl state tracker doesn't quite
obey this so for now just warn instead of assert.
Also warn instead of disabled assert when creating sampler views
(same reasoning).

Addresses https://bugs.freedesktop.org/show_bug.cgi?id=61647.

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