llvmpipe: Basic implementation of pipe_context::set_sample_mask.

llvmpipe: Basic implementation of pipe_context::set_sample_mask.

We don't support MSAA (ie, number of samples is always one) therefore
sample_mask boils down to a synonym of the rasterizer_discard flag.

Also, this change makes setup actually use the value received in
lp_setup_set_rasterizer_discard instead of reaching out to llvmpipe
upper layers to re-fetch it.

Based on Si Chen's draft.

With this patch `wgf11multisample Coverage passes 100%` on the UMD
D3D10 state tracker.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Si Chen <sichen@vmware.com>