gallium: new, unified pipe_context::set_sampler_views() function

gallium: new, unified pipe_context::set_sampler_views() function

The new function replaces four old functions: set_fragment/vertex/
geometry/compute_sampler_views().

Note: at this time, it's expected that the 'start' parameter will
always be zero.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Emil Velikov <emil.l.velikov@gmail.com>