gallium/gdi: use GALLIUM_FOO rather than HAVE_FOO

This matches what other targets do, and makes it easier to port to
meson.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>