i965/gen4: Fix GPU hangs since the program streaming change.

i965/gen4: Fix GPU hangs since the program streaming change.

This was tricky.  We were doing a use-before-initialize of
grf_reg_count, but the value usually got overwritten anyway -- when we
didn't have to do a relocation (typical), or on gen5 when we didn't
have relocations at all.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38771
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>