i965: correctly assign SamplerCount of INTERFACE_DESCRIPTOR_DATA

i965: correctly assign SamplerCount of INTERFACE_DESCRIPTOR_DATA

We were dividing by 4 twice.  This also papered over a bug where we
were neglecting to clamp the sampler count to the [0, 16] range.

This should have no functional impact, it only affects prefetching.

v2 [Kenneth Graunke]:
 - Clamp sampler_count to [0, 16] to avoid overflowing the valid values
   for this field.  Write a commit message.

Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>