agx: Require an immediate for `nest`

agx: Require an immediate for `nest`

There's no good reason to allow non-immediate nesting values, and this lets us
use the (smaller) mov_imm instruction without special casing. This matches what
Metal produces, so it seems like a good preference.

   total bytes in shared programs: 11720338 -> 11717310 (-0.03%)
   bytes in affected programs: 2341580 -> 2338552 (-0.13%)
   helped: 1385
   HURT: 0
   Bytes are helped.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24635>