i965/miptree: avoid uninitialized variable warnings

i965/miptree: avoid uninitialized variable warnings

GCC 8.1.1 is having a hard time identifying that the values are
properly initialized when used. In the 'memset_value' case, we pass
the uninitialized value to another function (that will use only if the
conditions match the initialization).

Just give enough hint to the compiler to figure things out. Fixes the
warnings

../../src/mesa/drivers/dri/i965/intel_mipmap_tree.c: In function ‘intel_miptree_alloc_aux’:
../../src/mesa/drivers/dri/i965/intel_mipmap_tree.c:1839:18: warning: ‘memset_value’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    mt->aux_buf = intel_alloc_aux_buffer(brw, &aux_surf, needs_memset,
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                         memset_value);
                                         ~~~~~~~~~~~~~
../../src/mesa/drivers/dri/i965/intel_mipmap_tree.c:1698:10: warning: ‘initial_state’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       if (wants_memset)
          ^
../../src/mesa/drivers/dri/i965/intel_mipmap_tree.c:1772:23: note: ‘initial_state’ was declared here
    enum isl_aux_state initial_state;
                       ^~~~~~~~~~~~~

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>