svga: null out all sampler views if start=num=0

svga: null out all sampler views if start=num=0

Because the CSO module handles sampler views for fragment shaders
differently than vertex/geom shaders, VS/GS shader sampler views
aren't explicitly unbound like for FS sampler vers.  This code
checks for the case of start=num=0 and nulls out the sampler views.
Fixes a assert regression in piglit's arb_texture_multisample-
sample-position test.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>