svga: replace sampler assertion with conditional

svga: replace sampler assertion with conditional

For TEX instructions, the set of samplers and sampler views should
be consistent.  The XA state tracker sometimes passes an inconsistent
set of samplers and sampler views.  Rather than assert and die, issue
a warning.

v2: add debugging code to detect inconsistent state.
v3: also check for null sampler in svga_state_tss.c

Cc: "10.0" "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>