gallium: add condition parameter to render_condition

For conditional rendering this makes it possible to skip rendering
if either the predicate is true or false, as supported by d3d10
(in fact previously it was sort of implied skip rendering if predicate
is false for occlusion predicate, and true for so_overflow predicate).
There's no cap bit for this as presumably all drivers could do it trivially
(but this patch does not implement it for the drivers using true
hw predicates, nvxx, r600, radeonsi, no change is expected for OpenGL
functionality).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
llvmpipe: fixes for conditional rendering

honor render_condition for clear_render_target and clear_depth_stencil.
Also add minimal support for occlusion predicate, though it can't be active
at the same time as an occlusion query yet.
While here also switchify some large if-else (actually just mutually
exclusive if-if-if...) constructs.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>