asahi/genxml: Overflow up to words when packing

So we can pack things that aren't 4-byte sized. Note this doesn't help
with alignment.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18623>

src/asahi/lib/gen_pack.py

@@ -26,6 +26,7 @@
 import xml.parsers.expat
 import sys
 import operator
+import math
 from functools import reduce
 
 global_prefix = "agx"
@@ -369,7 +370,7 @@ class Group(object):
             elif field.modifier[0] == "log2":
                 print("   assert(util_is_power_of_two_nonzero(values->{}));".format(field.name))
 
-        for index in range(self.length // 4):
+        for index in range(math.ceil(self.length / 4)):
             # Handle MBZ words
             if not index in words:
                 print("   cl[%2d] = 0;" % index)