pvr: amend num temps calculation when wg_size is not provided

Fixes: 7a32dc673ba ("pvr: add device info and functions for calculating ava...")
Signed-off-by: Simon Perretta <simon.perretta@imgtec.com>
Acked-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37724>
(cherry picked from commit e7c409cd29b1c4e58627b04cd725932d1a181d41)