nv50, nvc0: initialize ctx->sample_mask to ~0

nv50, nvc0: initialize ctx->sample_mask to ~0

Commit 95bf222603b (cso_context: Fix cso_context::sample_mask initial
value.) fixed the cso sample mask to be initialized to ~0. The cso code
is also careful not to needlessly call set_sample_mask, so we ended up
with the ctx->sample_mask never being set. This broke a number of
EXT_framebuffer_multisample piglit tests.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>