ir3: Allow tesselation to use all 32 varying slots

POS, PSIZE, CLIP_DIST0, and CLIP_DIST1 have their own predefined
indices, map's size should take this into account.

Fixes: 9e063b01 "ir3: Switch tess lowering to use location"

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7917>
(cherry picked from commit 22180137e9709937116f622185d027a108410236)