i965: require pixel scoreboard stall prior to ISP disable

i965: require pixel scoreboard stall prior to ISP disable

Invalidating the indirect state pointers might affect a previously
scheduled & still running 3DPRIMITIVE (causing page fault). So stall
on pixel scoreboard before that.

v2: Fix compile issue :(

v3: Stall on pixel scoreboard

v4: Drop the post sync operation (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Fixes: ca19ee33d7d39 ("i965/gen10: Ignore push constant packets during context restore.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106243