llvmpipe: Reuse llvmpipes LLVMContext in the draw context.

llvmpipe: Reuse llvmpipes LLVMContext in the draw context.

Reuse the LLVMContext already allocated in llvmpipe_context
for draw_llvm if ppossible. This should decrease the memory
footprint of an llvmpipe context.

v2: Fix compile with llvm disabled.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Signed-off-by: Mathias Froehlich <Mathias.Froehlich@web.de>