ilo: fix builder size checks for BLT buffer clear/copy

ilo: fix builder size checks for BLT buffer clear/copy

In buf_clear_region() and buf_copy_region(), max_cmd_size was set to 0.  If
either of the functions is called and there is not enough space in the
builder, the next ilo_cp_flush() will fail silently in a release build.

Replace magic numbers by size defines in tex_clear_region()/tex_copy_region()
for consistency and readability.