gallivm,llvmpipe,draw: Support multiple constant buffers.

gallivm,llvmpipe,draw: Support multiple constant buffers.

Support 16 (defined in LP_MAX_TGSI_CONST_BUFFERS) as opposed to 32 (as
defined by PIPE_MAX_CONSTANT_BUFFERS) because that would make the jit
context become unnecessarily large.

v2: Bump limit from 4 to 16 to cover ARB_uniform_buffer_object needs,
per Dave Airlie.

Reviewed-by: Brian Paul <brianp@vmware.com>