pvr: Fix packing issue with max_{x,y}_clip

pvr: Fix packing issue with max_{x,y}_clip

The spec. guarantees the framebuffer width and height to be `> 0`
but the same is not true for the render area.

Previously a render area of `0` size would wrap around due to the
`- 1` so we now check for `0`.

Fixes:
  pvr_packet_helpers.h:79: __pvr_uint: Assertion `v <= max' failed.
on
  dEQP-VK.api.pipeline.renderpass.framebuffer_compatible_renderpass

Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Luigi Santivetti <luigi.santivetti@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24329>