gallium: raise PIPE_MAX_SAMPLERS to 32

gallium: raise PIPE_MAX_SAMPLERS to 32

The previous value of 18 was motivated by having drivers that want to expose
16 samplers but also use some additional samplers for internal use. Raising
the value even higher isn't going to hurt that case.

On the other hand, some drivers actually use PIPE_MAX_SAMPLERS as the number
of samplers they expose externally, so raising this number above 32 is fragile
(because several places in the code use bitfields, and tracking down and
widening all of them is prone to miss some case).

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>