llvmpipe: Add missing LLVMGetGlobalContext() arg in lp_test_format.c.

llvmpipe: Add missing LLVMGetGlobalContext() arg in lp_test_format.c.

Fix build error introduced with commit
eedbce9c63a3f385908bdc8a69e8be98dd3522ff.

lp_test_format.c: In function ‘test_format_unorm8’:
lp_test_format.c:226:4: error: too few arguments to function ‘gallivm_create’
    gallivm = gallivm_create("test_module_unorm8");
    ^
In file included from ../../../../src/gallium/auxiliary/gallivm/lp_bld_format.h:38:0,
                 from lp_test_format.c:42:
../../../../src/gallium/auxiliary/gallivm/lp_bld_init.h:58:1: note: declared here
 gallivm_create(const char *name, LLVMContextRef context);
 ^

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84538
Signed-off-by: Vinson Lee <vlee@freedesktop.org>